I'm in a similar situation, about 33 feet of cable needed to reach from iMac. Did a bunch of research, decided it just wasn't going to work well enough to justify "cobbling long USB cables together with a repeater" for use with a nice DAC (like putting bald tires on a Ferrari). So have been limping along with a $6 Monoprice 35 foot optical cable with, surprisingly, almost no dropouts for the past year. (Ethernet is designed for longer runs, but few DACs accept it.) Some USB cables can be bought that are longer than 15 feet, but in general longer USB cables are frowned upon (few high-end cables are even offered beyond 2 meters).
Rather than spend thousands on a long (and compromised) hi-end USB cable, you'd be better off having a dedicated mini to get the most out of your DAC.
But the idea of having 4 Apple products in the same room (iMac, mini, iPad for control, and iPhone) and still needing screen/keyboard (or some kind of work around) for setup/maintenance drove me nuts. Finally decided to keep life simply by settling with my MacBook from the listening chair (8 feet) to back of equipment rack and probably a Halide HD that is affordable and highly rated (which has captive USB/RCA cables that can be ordered up to 15 feet long).
Hope this helps. Please let us know what you decide to do.
