"The Alesis-IO2-Express and other interfaces I checked (Focusrite, M-Audio) are USB powered. IS USB power really adequate for 1 or 2 x phantom power + line out + processing?
Do all audio interfaces let you hear what you will be recording onto (I.E. the mix saved on the PC) and also monitor what you're playing, both in the headphones?"
In a word, err...Yes! You have however brought up a sore point! Phantom power delivery IS compromised on some AIs (and not a few mixers). If the NI KA6 has a flaw it is a rather wimpy spook juice capability tho' it run my two P150s fine.
Very few budget capacitors AFAIK are all that fussy about phantom volts? I have certainly not read of any issues but I doubt that many AIs in the sub £200 bracket could meet the 10mA specification, especially those that are completely bus powered. Fortunately, as I said.it rarely seems to matter.
I did in fact buy two, 2 chan' phantom power boxes some months ago and was incensed to find that they were WELL short on V.I delivery. I have complained bitterly to both the company and the vendor but have heard nothing back. Fortunately, although they are nowhere near specc' they serve in my situation. Caveat Emptor!!
Monitoring? You will have no problems with any of the AIs mentioned thus far. Note, things can need sorting a bit in software.
Dave.