Will this work? (M-Audio FastTrack-like unit, and USB content/questions)

I've been looking 'round the netz, and have decided that something like the FastTrack USB might just work for what I want. But I don't entirely know, so I'm asking here I'd prolly get whatever when and if I get a refund for my ProDI (bum unit, but pretty much 100% likely not to work and do what I want [sigh]).

I've done a little experimenting and come up with a setup that will allow me to use a unit like the FastTrack with an FX loop type amp setup.

Basically I'd split the signal at the send jack (1 mono -> 2 mono type thing), and have 1 patch cord going to the return, and the other going to the input on the interface.

Like I said, I've experimented with this and get no noise, if my laptop is running on batteries. And thats the thing. I want to run it on AC power. But, I get a ground loop

What I am looking for is 2 things answered:

1) is there a box that has attenuation all the inputs (I think the FT only has that on the XLR, not the 1/4" ). Im gonna be feeding it line level ****, so I don't thing something like the SteathPlug would work.

2) Would I defeat the ground loop by using a USB interface. I don't really want to get something like this and find out that I still get noise if the LT is running on AC