Hi all, first post. I bought the THX AAA 789 on the basis of Amir's measurements. It's being fed off of the RCA outputs of an O2/ODAC (Massdrop version, purchased for $155 off of Craigslist, also on the basis of Amir's measurements). Sounds great so far, and it can drive multiple headphones at once - very interesting when comparing The Sennheiser 6XX and Hifiman 4XX.
(Subjectively, ) I've preferred the 6XX, but when being driven by the THX, the Hifiman 4XX sounded much better than it did via the O2/ODAC. If anything, I feel like I'm up against the limits of recorded music (and my ears), rather than some specific weak link on my desk. I would say the THX has greater bass extension than the O2/ODAC. I don't drive them anywhere close to their limits: about 12 o'clock on the lowest gain setting for either amp.
@Memoryerror , I _may_ have heard something similar to what you're describing, but I think it also could have been my imagination. When listening to streaming media over Spotify/Tidal/Roon/etc., it's also really hard to blame the headphone amp rather than the software, Mp3/Ogg compression, DAC, or anything else in the line.
As an aside, my computer speakers are an old set of Klipsch THX 2.1 speakers. The THX 789 manuals says not to "drive loudspeakers" with it, but the Klipsch THX speakers are powered: is there any issue with using the SE out from the THX 789 to the audio in on the Klipschs? "Theoretically," it sounds pretty nice, although it's a chain with like 4 volume controls in it.