Well thanks for the input.. i dont know how much noticeable sonic improvement the audinst will benefit from the increased power anyway. I'm still quite skeptical over this.
From what I read, both the audinst and tube-dac11 have no problems synergizing with the HD600s. The T-11 costs about $200 more but I'll be getting the tube output (and the thought of swapping the tubes kinda excites me) and the cirrus CS4398 (I'm assuming its better than the wolfson 8740 implemented in the audinst?).
It's not necessarily increased power, but better quality power. As you move higher up the ladder for DACs and amps a lot of what you are paying for is increased power supply quality. In fact a lot of folks are of the opinion that for the most part we are chiefly listening to the PSU and everything else is secondary. Not sure I go that far, but it is an important aspect.
The WM8740 and the CS4398 are both quite capable chips. It's the way they are implemented that matters most. The TubeDAC-11 is just a more ambitious design. But the Audinst is nice for the asking price, so you have to decide what you want to spend.