I bet the HPA4 has a high quality discrete preamp stage or some type of discrete output stage that has an influence on it's presentation vs the cheaper THX amps. Any idea?
Not an unreasonable guess, but I could find nothing suggesting a discrete output stage after quite a bit of searching. In fact, it seems that the feed-forward error correction in the output stage would rule out a discrete components approach. I know that in general Benchmark prefers opamps because of their tighter tolerances and thus greater consistency and that they rely of that in order to lower THD+N, so I suspect they would avoid discrete components whenever possible. But, that's me speculating.
Personally, I would suspect that the higher quality power supply and very high damping factor would be the contributors to the why the HPA4 has a richer sound over the 789 and similar amps. The HPA4 just has a firmer grip on the power it delivers and so can handle delivering much bigger swings in power output without under-delivering the power or overshooting the signal. That would help it render that more natural decay to notes without artificially muddying them and also outperforming the lower priced THX amps on slam or macrocontrast. But again, that's me speculating.
What I know for sure is that the Arya is a great match with this amp. The HPA4 doesn't do it any favors when it comes to changing frequency response, but the extremely clean and controlled amp does help it make that frequency response sound its best. That might be confusing, but maybe some examples would help. The HPA4 does not increase the quantity of the bass, but it is able to keep that bass at the same level no matter how complicated the track. If bass is in the song, you'll hear it. Moreover, with the increase in bass slam or macrocontrast, the bass feels more visceral and easier to pick out, even if the headphone presents it at lower volumes. So, not more bass, but bass that is easier to notice. That's also true of the 2k dip and vocals. Likewise, the amp does not render the peaks at 3-4k or at 6k any less elevated, but its superb resolution eliminates any extra resonance, blur, or glare that might aggravate those peaks. Ultimately, it's not going to be enough to 'fix' these headphones for someone who just hates the FR, but it does give these headphones the best shot of being appealing. For some people on the cusp of liking them, pairing with this amp might be just what they need. And, if EQ is needed, it's way easier to dial that in perfectly when you're using an amp that doesn't introduce any problems or frequency adjustments of its own.
