Really engaging amp, with plenty of power. The gain switch lets you use this with a clean background even with IEMs. I've used the TFZ Secret Garden, Noble X, Hifiman RE600s v2, Noble EDC Velvet, none of which had problems; no background sound, and all sounded pretty good.
For full size headphones, I really enjoyed almost all of them. (Audeze LCD2C, EMU Teak, 1More triple driver over ear, Sennheiser 6xx, Sennheiser 58x, Fostex Tx0, Hifiman HE400i, Hifiman HE4xx, Audio Technica M40x) The amp is relatively neutral, and provides a sterile environment for you to really compare headphone drivers. I think it also provides a solid synergy with most of my headphones such that it's more enjoyable to listen to music on this amp than others in my stable: Aune X7s, Vali 1, Lake People G103, and I prefer this to the V200 I had. I think it really punches above its weight class for features and capability.
My one minor gripe is that the socket for the headphone jacks feels odd. The 1/4" and 3.5mm have a weird gritty feel. They both work fine, but I feel a little extra friction when inserting headphone cables. Literally the one thing I can fault on this machine so far.
Suppose I should add, used in the following chain:
PC->Bifrost Multibit->Onkyo TX8255->THX AAA789
PS4->Astro Mixamp Pro->Onkyo TX8255->THX AAA789