another thing to note on with usb sound.
for a while i was hearing minor clicks and crackles when i would turn the knobs on my midi controllers (uc-16 & oxy8). but i have my first live gig coming up soon and so i made it a priority to resolve the issue.
i originally had the quattro connected to a port on my usb 2.0 pcmcia card, with the two controllers connected to the built-in ports on my laptop. what i ended up doing that fixed the problem was getting a usb hub for the controllers and putting the quattro on one of the built in usb ports. now i can turn all the knobs i want without any crackling.
the moral of the story:
usb via pcmcia = no good
or
usb audio is temperamental stuff, so perhaps just try reinstalling the thing on a different port if possible.