I am curious about how much better the amp is (if at all) in the HM-901. I really really don't like carrying stacks around either, so I use the HM-801 despite its compromises. I also don't like the idea of running an amp into an amp - most of the portable DAPs have no digital output.

What makes me nervous is Fang's decision to use the Sabre. Seems like a very odd choice going from the "analog" sounding PCM1704U/K to the very "digital" sounding (most of the time) ESS DAC that supposedly is a pain in the ass and very difficult to implement properly.

I knew that the supply of 1704s had dried up, I just was hoping to see something like the WM8741 or the AKM4399 in there instead. The Sabre has a reputation of sounding rather sterile and cold unless done exactly right. I know it's a great bullet point to have on your marketing sheet that you have the "awesome" ESS 9018, but that doesn't make it the best choice.

I guess we'll just have to wait and see. The price has gone up several hundred more over the 801, so it had better be good.

Well they do seem like pretty dramatic differences on paper. Shouldn't be extremely hard to tell them apart.

I can say that with quick ABX, it was really hard to tell the filters apart if at all.

I can't say I've heard the PWD2 using the X or Y filters 'cuz I just heard it at meets or so with whatever filter or configuration it had. But Marv was clearly not alone in having a hard time figuring out the differences. Alex was also open about having a hard time telling the filters apart: http://www.head-fi.org/t/492556/perfect-wave-dac-owner-unite/825#post_8832967

FWIW the digital filters are likely low pass, and by going from linear to minimum phase, the high frequencies are likely the most affected. How much difference there is in paper between linear and minimum phase might depend on implementation (how many taps, what type of filters were used, what windowing was used to cut the filter, what is the filter 3dB frequency, what sampling rate the filters require and so on.) Additionally, other things in the chain may dominate... Like the analog filter in the DAC, the amp... Honestly, I don't know how difficult it would be to differentiate the effects of filters in the chain unless I actually try them. Based on the little that I know, maybe high frequency stuff is easier to catch. If it's just the air, I can easily see me not being able to tell them apart (I don't think I can hear past 15kHz.) But who knows...

I hear a difference between the different filter settings on my Gamma2 but none of them sound bad, and anyone with a strong preference is full of it.  With the gamma2 anyways.  I just set it to the one that sounded a bit better to my ears and then whatever

I think the thing most audiophiles need to wrap their heads around is that just because there is a difference, doesn't mean it matters.  If you're not sure which is better, it doesn't matter.

differences in the filter settings on the gamma2 are subtle, more prevalent over a longer listening session, and with HPs with a more upper-mid/treble emphasis.