I'm using a Fosucrite Scarlett 2i2.  USB 2.0.  2 in/2 out at 24/96 in ASIO.  Direct sound/wav out/WASAPI is only 16 bit.  It's USB powered but I don't see and noise show up in loopbacks.  It's super clean from the balanced output.

Hmm, decently priced. I like.

Are you using an amp on the balanced out? How's the headphone out on that thing with the T50rp?

I made some 1/4" TRS to RCA that just connect the ground and positive and run it to my O2.  The head amp isn't great but it's not terrible.  It's pretty clear but it has some output impedance that messes up IEMs and it doesn't have much power for a modded T50RP that's had it efficiency knocked down.  It would work better with other stuff though.

i just use things from echo. i think the audiofire 4 has phatom power/mic preamps and so forth. i forget. but echo is really good but downside is not lot of non-mac computers support a good firewire host and so forth so you have to make sure the motherboard in your current computer or your laptop uses firewire. firewire usually is overall faster and more effcient than usb as well.

don't know much usb products except from behringer and couple from yamaha. behringer ultra is wall powered and 24-bit/192 support. has nice amount of inputs and outputs as well. you can also run headphones straight from the interface balance outs if you wanted to but due to the outputs high output impedance might run into issues maybe. but voltage wise t's more then enough for just about any ortho or dynamic at +4dbu setting @1.23v rms. you can also use balance outs to drive power amps if you need to cause 1.23v rms should be fine for most power amps since lot of power amps can reach full output within couple of volts but it depends really.

just make sure not to overload the inputs if your hooking balanced outputs or inputs to consumer gear(line level stuff) or if you're already using a preamp for the power amp. you might notice higher volume but it can cause distortion much quicker and by thati mean the sond will become less clean and start to seem less clear and stuff. you'll know when you hear it when switchng between -10dbv,+4dbu,ect. if the interface allows it.

Ops. Missed the replies. I ended up buying for the Focusrite unit, and it arrived yesterday. It is quite nice. I especially like the clipping indicators. My new measurement plate also arrived yesterday, and I've been playing around with both of them, trying to get some consistent result going. It really is harder than I thought it would be. Ganna get some more foam to try to kill the artifacts in the treble region.

Have not been running a dedicated instrumentation amp from line out, but I've got something on order for it.

Well those 600 ohm AKGs barely draw any current anyway but working with the T50RPs is surprising...