Low gain. Volume position on pot is 2 out of 10. Using ER4S. Which is as loud as talking levels. 3 out of 10 is loud. Anything more than 3 then you're deaf, that's all I can say. Lol. Can't say anything about the output power but all I can say is it powers my Ultrasone PRO750's much better than my Headsix and my Minibox-D
After 3 or 4 days use I'm getting more and more impressed with this little beauty ( and I do mean LITTLE!!) I've switched back to Ety 6i's instead of UE5's and 5Pros. Both UE's hiss somewhat when plugged in - even at low/mid volume levels. Not a huge problem to me -when music starts all is fine. But Ety's are quiet. They take a bit more driving- I tend to have volume at approx 4or5 out of 10 - whereas the UE's are at 2/10 and almost too loud (even on low gain) The bass boost isnt huge - but agree that the T4 adds a nice bit of bass to Etys.
Physically the tiny size is perfect - at the moment I have a 3rd gen nano back to back with velcro dots and a qables LOD. I'm tempted to upgrade to a16gb 4th gen nano - that would match the T4 perfectly!! Same tiny width - possibly thetiniest player/amp combo ever!!