First of all, as a professional user I must say the sound is transparent - very good, but only in seven out of eight output units ;). No biggie. The eighth one (the one closest to the power switch) has some very clearly audible buzz in it.
The passthroughs are clean, and they are properly passive: the balanced signals go to my monitor controller even if this unit is powered off.
Also, this amp is freaking loud. Radically louder than Presonus HP4, for example, and that one isn't exactly from the quiet end, either. Don't get me wrong, it's great that this is able to drive even very demanding cans. Still, it's... SO loud I wish there was a way to at least calibrate the nice meters based on your phones and effective listening levels :)
In other words, even when using moderately high impedance 250-300 ohm headphones, at a comfortable listening level, the output meters are barely hitting halfway. On 20-40 ohm phones you are working pretty much without the light show (hehe) unless you wish to hear absolutely immense levels.
All in all, for the purposes I got it for, it absolutely delivers! Clean and loud headphone amping.