Thanks, I was wondering about the output power (and other specs such as R-out which is eay to deduct).
Asssuming the endstage doesn't have a current limiter and has a low output R the Vali will put out 500mW into the same 50 Ohm load, that is 12dB more headroom !)
For 300 Ohm HP's the output is about 80mW (6mW for the focusrite)

The increase in distortion is typical for a triode design without any feedback.

Given the noise level I would not recommend the Vali to those using IEMs or other high efficiency headphones.
In the old analog days I could live with some background hiss but don't think I have to put up with it any more in these days, except for hiss in recordings themselves which cannot be avoided unless music is all electronic or noise gates have been used.

The noise floor is still there with the HD558s, but music masks it pretty well. I could understand if that puts off some people.

There is definitively some increase sense of fidelity here. Not sure if it's the amount of headroom, and/or the small signal versus large signal distortion differences, or what. There is no classic clipping farting stuff for sure. Things (bass through treble) seem to have more perceived detail for some reason.

What kind of load with the tests?

No load. Just headphone out directly to line-in.

EDIT: Just went through the manual and the line-in is about 10 kohm, but haven't measured it.

I can try making my own loads later. Seems the Vali is relatively low output impedance though.

LOL! The Focusrite manual claims 0.002% THD+N for the line-in and 0.001% THD+N for the line-out (balanced)... I got -92 dB (0.0025%) total. Unbalanced it seems its more like -72.7 (0.023%) though... It's just a relatively good interface for the $. No dScope, Audio Precision, SR1+ (Schiit's measurement rig) or anything like that. Probably more suitable for headphone measurements than amp characterization.

Thanks for the measurements, ultrabike. I had some fun using RightMark on the vintage Sansui receiver I got a couple months back. Not horribly scientific...had an Auzentech Bravura at the time and just did Bravura LO (cheap cable) -> Sansui aux-in -> Sansui HPO (stock T50RP cable) -> Bravura LI. I set the volume to match what RightMark wanted, and it wasn't too far off how I'd set the volume on the HE-500 or Mad Dog with the receiver. With that I got:

Frequency Response: +0.14,-0.43 (Not as bad as it looks. Less than 0.2dB variation from roughly 40Hz-10Khz. Rolls off closer to 1dB around 20Hz and 20KHz, hence the numbers here.)
Noise: -59.5dB
DR: 59.5dB
THD %: 0.030
IMD + Noise %: 0.410
Crosstalk: -51.9dB

On paper, that's really not so great compared to a lot of other headphone amps. I see the Vali doesn't do so great either, though mostly better than the receiver. Yet, I'm hearing a lot of good impressions about the Vali, and I've heard other respected members on this forum mention that vintage receivers can sound really good with some headphones (generally planars). I, for one, really like how my receiver sounds. I'm not trying to get OT here, but more using my experience to relate to your impressions of the Vali vs. the Focusrite and how it would seem counter intuitive based on the measurements.

Now, these numbers can only tell part of the picture. The measurements of my Sansui were particularly revealing once you took a closer look (I posted them in another thread a while back). For example, while the noise floor might be a bit high, the graphs show the noise floor is actually quite clean and low at around -94dB from 2-3KHz and up (likely better in reality due to the limitations of my gear). The main problem with the noise floor is a large spike at 60Hz and associated harmonics. The same was true for THD and IMD. And with the FR, you can see it's relatively flat for a device that has no bypass for the tone control knobs (setting them dead-center gives a worse response, tested in RightMark - yeah, the amp probably needs some restoration).

So, in my case, the "summary" numbers from measurements weren't fantastic, but there was actually much more to it when you looked at the graphs. I've already since upgraded the RCA cable I run from the sound card to the receiver, and that alone dropped the noise and distortion peak at 60Hz by -6dB or so (verified in newer RightMark measurements I never posted). But, besides that, my measurements showed I probably am seeing a minor ground loop problem and/or the PSU in my computer is not the best (could be motherboard or just my system as a whole, which is a gaming rig after all). The issues on the measurements might have even been exacerbated by my measurement setup and techniques, mostly in that it was a closed loopback measurement. And, also, when you consider this is a 40+ year-old receiver that might have had little to no restoration work done on it (all I know is the known faulty boards were replaced) and combine that with other possible issues and my less-than-optimal measuring techniques and equipment, I think there are many more positive signs than summary measurements indicate.

Again, I know I'm off topic, but I think you should post the measurement graphs as well once you figure out how best to measure the Vali. In my case, they were much more informative than those quick stats RightMark gives along with them. (Though, yes, the situations and hardware are quite different outside of the fact that we're using them to drive headphones. I also understand that other graphs of the Vali were posted earlier, but I'd like to see how ultrabike's graphs would turn out in relation to his posted measurements.)

Well, I know that's just one of your cases, but that looks good enough to me to not worry as long as I like what I hear.

Welp. Got about 15 ohms for the Focusrite  , and about 10 ohms for the Vali (which is close to what Jason estimated).