I've only had it for a day and it sounds freakin awesome!
I've had a Cosmic Traveler in the past, which was broken by somebody, and then another HeadRoom amp that I didn't mesh with (returned),
most recently I've had the FiiO E3 and E5 amps, working my way back up to better amps as I go, and now have been able to afford the T4, and boy am I glad I did!
It sounds really quite good, especially with the iBasso CB01 Silver Flat Cable Interconnect (and Hopefully with the Mini to Mini Straight Black Dragon from Moon Audio that's on its way)!
It really is quite good at resolution and instrumentation; and bass is solid! I'm very pleased, and shocked at the quick order-to-fulfillment timetable.
Thanks iBasso!