All good questions and thanks for the interest. The Miditech soundcard, the laptop and all the connections to/from them are not normally connected. All this stuff is for making measurements with REW to setup up the system. This is a seven way system. 1x 13.5" sub, 2x 8" midbass, 2X mid range and 2x tweeters. Without all this connected the system itself is fine (maybe some EMI on the speaker wires but this was unavoidable, for now its fine). There are 3amps. 1x for midbass, 1 for sub the other is 6 channel and does the front components, all JL audio XD series. Theres a signiture pic on AV Nirvana link above. The bigger picture is explained on that thread.
Half these problems, what this thread was for, is the 50hz hum noise when I plug the laptop into mains when my measurement gear is in use. I realise that Im gonna have to start trying USB isolators on the relevent cables which should solve this problem. The MiniDSPs themself both utulise a MiniDC isolator which powers the 2.5watt devices but when any other galvanic connection is made (like USB) it will pick up noise. Yes there is also noise introduced from the soundcard via 1) the USB connection and 2) the actual analog side of it, its rubbish really but I don't have the experiance to think of an alternative.
A simple USB mic, like the Umik, could be used but this doesn't use loopback timing for the impulse measurements, relying on REW's acoustic signal instead. I don't like that because its unreliable at the high frequencies plus Windows can be buggy with it.
I get overwelmed with inputs and outputs let alone what channel the loopback is on and don't fully understand ASIO yet, I just wing it till it works.
The MiniDSP has its own ASIO driver inside but don't ubderstand how / when this would be used. I hope this clears up most of it, fire away if you have any suggestions. Thanks
ps yes the miditech is bus powered, not sure of the current draw tho.