Hm... It's not clear if they talk about listening through the DAC section or just AMP, there might be the Sinergy problem between HPA and different headsets and the burn in thing that generates so much argue, and then the human factor, tastes etc. I don't know what to say. I use mine with HD800 and sometimes with JBL S700. I'm totally good with it! I'm no expert but I use only WAV up to 192khz and sometimes I can feel when for instance a track has bad masterization or bad compression. It just sounds screechy or congested. Many times I load the tracks in Adobe CC to see what's going on and you can see if it's compressed and mastered like a big chunk of wood with all freqs up on the same level and then cut like with a knife.
Yes, reviews are unclear. All that IS clear is how the HPA-2 sounds to you (think you're talking about the HPA-2). As Duke Ellington famously said, "If it sounds good, it is good."
My HPA-1 has no DAC. My DAC is the Audio GD NOS 19. That makes evaluating the HPA-1's inherent sound a little easier: I used it as a preamp and sometimes as headphone amp. I have other headphones amps, one of which is also a preamp, so I can compare the HPA-1 to other quality units. For example:
HPA-1 vs Burson Soloist (the big/powerful one, not the SL model)--as both a preamp and a HP amp: The Burson is larger, clearly more powerful, but much harder to actually use because it's stepped potentiometer has relative few steps (21 or 22 total). The Burson has slightly greater extension at both ends and the bass hits like a hammer. It's probably "technically" better than the M Stage, but I actually prefer the HPA-1 for its easy-to-use volume pot and listener-friendly qualities of warm and slightly elevated bass.
HPA-1 vs Lake People G109A (as HP amp): These 2 sound rather different, but in very subtle ways. The G109A is more powerful, relatively flat, with a hint of warmth and smoothness. It doesn't exaggerate treble (any frequency, really). I call it a "truth-teller," but it's not fatiguing at all. The HPA-1 is slightly looser, less of a grip on everything, but somehow is fun & friendly--a somewhat warm, bass-rich sound.
The HPA-1 should fail in comparison to these more powerful headphone amps, but it doesn't. It always sound good and has plenty of power for any of the headphones I've used: I use it on the lowest gain setting and can't get the volume pot up even halfway w/o going deaf.