So you plug these in to the USB and it presents itself as a sound card to the computer?
yeah, basically. I like using the optical out so I don't have to fiddle with selecting a soundcard or stuff like that, but the premises is basically the same.
I would much prefer to use the USBpre2 as both a DAC/Headamp if I could get away with it... I wonder what other headphones might not have the hiss problems with it....
That was totally my aim, but I couldn't stand the hiss so I always used an outboard amp until I got my LCD-2s. A general gist on keeping low-impedance but low-sensitivity is looking for orthodynamics instead of straight dynamic coil headphones. Audeze, HifiMan, and maybe one other make nice ortho cans, it's really a niche product anymore. Otherwise most low impedance cans are also fairly sensitive, and SD says the usbpre2 doesn't play as well with high impedance cans. While orthos are not a guarantee, it's a good indication that they will need lots of current and thats part of the problem with the usbpre2; it has an over abundance of current. Also, look for an impedance over 40ohm and under 100, thats the other thing. Anything under 32ohm and I suspect you start to get the hiss. This is all ballpark stuff, nothing hard and fast.
Having an outboard amp wouldn't be bad, I'll admit, I'm still tempted by the schiit asgard.
headphone.com has response graphs** that you can get a guage of how bright/dark something will sound as long as you can rationalize the graph they have for your own headphones.
** IME, they are approximate. More of a guideline but they can still be informative.