Hello,
Not to say some people don't hear differences with their USB cables or their lengths, but for me I've been using a 15 foot cable with the RWA Isabella, it is either a Belkin or Monster (I can't even remember now), from Fry's... it was the only cable they had at the max length of 15 feet.
I couldn't be happier, the system sounds great -- detailed, full, deep, all the adjectives. For a little while I was worried that the longer cable (or a "non-audiophile" USB cable), would introduce more "jitter" or something, given all the postings from other people on various sites... I dunno, I don't think I am hearing it... but then I'm probably deaf...