All I can add here is, if Lamdba was all Stax had to offer, I would never have become owner, even less sticked to it over the years. I never warmed up to these guys (although I mostly listened to the couple of gens. before X07 so maybe things have changed a bit). For me, Stax starts and ends at the Omega style series, including 009.
arnaud

This, very much so.

I had a 404 for a while. I liked it at first, then got sick of it, and quit stats for a few years, before coming back with the 007. There's no contest - the 404 sucks in comparison. I may even say "sucks period" but it's been quite a few years since I heard one, and I may be remembering the bad points more than the good. But, hearing your take on the 307, especially with regards to the artificiality of the mids and highs, reminds me of what I thought of the 404 a while back.

I also don't think there really is such a thing as "stat sound." Yes stats share some sonic characteristics, just as dynamics share some sonic characteristics, but I've heard enough and owned enough stats to say that they can be pretty different from one another. I agree with the relative lack of macrodynamics in a lot of stat-based systems (not all though), but then again I also can't unhear the inherent graininess and lack of low-level detail in most dynamic systems which prevents me from getting into most dynamic-based rigs (but again, not all).

I'd pick the HD600 over any Lambda (though I have a LNS lying around that needs fixing up which I'm looking forward to hearing). But, I'd pick a high-end 'stat over everything.

Crap, more fuel in the stat vs. dynamic debate - not my intention. I go back to lurking now  Nice take on the 307 and it reminds me of a lot of what I heard in the 404.

Stax 307+323S versus Koss ESP950+323S (Impressions of E90 amp included versus 323S)

Note:  On certain tracks, the 323S seems to clip or inject static noise into the 307 and 950.  So bear in mind impressions were with a defective review unit (Stax 323S)

Stax 307 (General signature)

These have a weird screaming resonance in the lower treble that bothers me with certain female vocals like Stevie Nicks and Madonna.  Keep in mind I listen around 80-90dB. Some tend to think dynamics have grainy character to them. In the case of something like the 307, I think some Stax lack texture.  These phones have a very plastic sounding timbre to them that sounds smooth at first till you realize it’s a coloration being forced upon all tracks regardless of the recording.  This may sound more pleasing to some, but it makes it more difficult to be critical of what actually is happening from the mastering or studio end of things.  Overall clean and fast sounding (apart from the amp distortion).  While signature is overall very smooth, I find a troublesome treble peak in one of the worst possible places for me and the bass is severely anemic with very little if any impact or slam.

Tracks:

Cannonball-Supertramp

Keyboard, pianos and guitars sound rather plastic and synthetic. Bass is quite limp in the mid and upper bass regions.  It is present but simply too anemic.  Everything sounds more closed in than the ESP950 or HD800.  The Koss 950 just opens up and allows the music to flow with ease by allowing air and space between instruments.  You can hear trailing notes decay and terminate without running into other instruments or just being compressed into each other.  The 950 has superior imaging and separation to my ears.  It also has a more natural and superior timbre overall allowing better tonal contrast between instruments especially in the 307’s dead zones (bass valley and lower treble peak).  Extreme extension could improve as well.  The 950 sounds more relaxed to me but also more resolving, open and natural.  The 307 has a sonic coloration that turns music into something that sounds like a children’s soundtrack produced on a plastic Hasbro device.  Don’t know how else to put it.

However, the 307’s smooth coloration can make grainy vocals on tracks like this sound more pleasing, refined and forgiving.  This smoothness is what gives the 307 an overall more refined signature where the 950 sounds more rough and gritty, almost lower-fi by comparison.  So those who weight this quality over all the others I mentioned the 950 excelling at should consider them.  For me, it’s just too much of a good thing and simply doesn’t provide enough gratification for the price point and promise of the supposed inherent technical advantages of a e-stat compared to other similar drivers, planars or dynamics.

Lucky Star-Madonna

The 307’s definitely sound light in the midbass.  The ESP950 actually give proper body and weight to the midbass filling out a more balanced frequency response.  307’s make Madonna’s vocals and this song quite fatiguing for me.  Just too bright.  Putting 950s back on…everything is much better, natural and balanced.  I’m ending this track evaluation after 2 painful minutes…
That said, the 307 does sound better and more pleasant with other vocals overall than 950.  More continuous and coherent vocals are silkier but the 950s resolve textural gradations in vocals better but add too much grain by comparison.  So in the battle of overly smooth versus overly grainy vocal stylings., the 307s win in that department.

Little Amsterdam-Tori Amos

Pianos are just too thin on this song. The 307 is missing a decent amount of ha rmonics and I would not recommend these to people that get great satisfaction from the complete resolution wood enclosed acoustic instruments (yes I know pianos are ‘string’ instruments).

On this track, 950 vocals come off a bit too hollow making them overly dry compared to the 307.  This obviously is what is adding to that sense of vocal grit or grain making them sound too ‘crispy’.  Once again, the 307 is more pleasant for vocals.  Good thing Tori Amos is closer to Demi Moore than Stevie Nicks.

On the E90 versus 323S

People say the E90 is good enough.  Get’s loud enough and all that jazz.  Well is it?  I don’t particularly like the Stax 323 either very much for the money especially once you’ve accustomed yourself to higher end amps like the T2 and Leviathan, etc.  Heck, even my old Super7 was more technically resolving and microdynamic than the 323 for close to the same money and wasn’t tied Stax.  Regardless, let’s find out as I have both on hand.

E90

Bass is quite soft.  Vocals are very upfront and slightly shouty.  There’s an ever present grain over everything making tracks sound sort of MP3’ish (like lower resolution).  This trait combined with the signature of the 950 is not the best combination possible.  Instrument placement is fine but imaging is very fuzzy and ambiguous.  The E90 makes the 950 sound like a stock Oppo PM1/2 in that regard.  However, it has much better air and spatial separation than those planars, just happens the edges of images sort of bleed out like a water color painting.  It can’t quite close the frame on each instrument and complete the sonic picture.  The E90 is also quite laid back and relaxed sounding in overall signature.  Not a lot of macro dynamic punch and speed, or inner resolution to suck you in.  This combination would be more for the less technical and more relaxed listener content to not spend a dime more than they have to.  It plays music, simply nothing special or truly competent about how the E90 sounds.  But if this is you, is there something wrong with your iPod or Sansa? 

Stax 323S

Much smoother, more articulate microdynamics and better resolving power than the E90.  Cleaner and clearer background.  Images are more precise and complete as well.  Still a bit boring for me dynamically and in resolving ability compared to my other preferred amps but the 323S is simply superior to the E90 in every way imaginable.  Even if this unit is defective, it’s still clearly more competent.

Conclusion

Overall on the phone side, the 950 is simply more balanced and resolving than the 307 without any weird treble peaks or bass valleys.  I simply find it overall more natural and more competent in various technical areas.  However, if you prefer to have a smoother, more refined signature that makes all of your bass deficient and depressed female vocal tracks sound better, the 307 wins hands down.  If your music has ballz or its female singers aren't dead or locked away in an asylum, the 950.  On the amp side, the E90 can run off a wall wart or batteries but that’s about it.  The 323 even steals its thunder by including its own separate L/R channel pots as well.  But hey, the Koss system can be had for $600 new in box and has a lifetime warranty.  All Stax owners wish they had anything remotely close to a lifetime warranty and decent reliable service outside Japan for the price they pay.

In the world of Stax, there is simply nothing better than paying a lot of money to be treated like crap.  That's how the cool kids do it!! 

Thanks... Very insightful. I hope to be able to give my own impressions soon ( Koss ESP950 , KOSS E90/ Stax SRM1/mk and Stax SR404) 

Thank You.  To be clear I do have two vintage Stax SRX mk3s that I like.  I also LOVE the mids on the original Omega and think the 009 can be quite special when driven properly.  Overall I do find the more modern Stax e-stat sound combined with the 'status' and relatively poor pricing and service to be off putting to me personally.  I just feel the need to offer a counter balance to such disturbances in The Force.

Sorrodje, note there are differences between the 404 and 307.  The impressions here aren't exactly compatible with what you have regarding those two HPs but might give you a general idea.  Think HD650 vs HD600.