http://www.head-fi.org/t/709479/multi-review-hifiman-hm901-fiio-x5-sony-zx1-hisound-studio-3rd-anv-iphone-4
Please read this thread if you have not already.
Provides opinions of some very well respected head-fiers on DAPs.
I recently did a double blind test with a friend.
We tested HM-901 (standard and balanced amp card) , AK100 Mk2, AK120, X5 & C5, Ak100 & Gloveaudio A1, DX50 with Shure SE846, UE900s and JVC HA-FX850. As a reference set we used a HD800 on a HDVD600 with balanced cables.
Conclusion was the same as in the thread you linked. Slight differences, but so slight they are almost irrelevant. Funny enough in a blind test with an input switcher, the AK100Mk2 (with FW2.41) and Ak120 were indistinguishable from each other. Surprisingly. Adding the DX50 to the mix was shocking too, we couldn't tell the difference.
Adding the HM901 to the mix we could hear a better soundstage and the A1 added a bit more dynamics and even darker background to the mix.
Again, we did all the tests with your headphones in your ears, volume matched outputs, same song playing (HD tracks demo songs in FLAC) switching back and forth (by the other person) randomly between up to 4 players at a time.
If you are not a musician (like the poster in the other DAP shootout) it's so hard to compare anything if you have to disconnect and reconnect your headphones etc. so using the input switcher is again removing the "memory" issue that you have to memorize how something sounded, because you can switch back and forth immediately with the same track playing.
So if I hear people musing about how firmware upgrades change the sound so groundbreakingly without testing them side by side, I just smile....the differences are microscopic at best.
Blindtesting is always an eyeopener as is using an input switcher, incredible how we fool ourselves in exaggerating the differences we hear or tricking ourselves in hearing differences that simply aren't there.
So our conclusion overall was: we did like the 901 and the Gloveaudio best, however by such a small margin that we would have been happy with the sound quality of any player. And that includes the $199 DX50 - after spending thousands of dollars we were expecting more. So we will continue using our expensive players as there were (at least in some) a few improvements. But would I buy it again? Probably not.
Spiderman was testing too.
