Just bought a Focusrite Scarlett 2i4 and am having the same problems. Thought it was the mic, then the mic cable, then the interfact, then the cable from the interface to the (new Fusion Drive) iMac, then I find out everyone is having the problem with Mavericks and audio interfaces. This is completely f*cked!
I'm presently congfiguring a Mac Pro (in fantasy land) until its released. I've filled my USB 3.0 ports, and have two TB ports available. I hate hubs. They clutter the workspace, they invariably fail, and they typically cause more problems than they are worth. With that, i have an ungodly collection of adapters and external devices with thru-ports. Seems to me, there is a market here for TB to USB 3.0 adapters, ESPECIALLY ones with a thruport. I wonder if the hold up is for TB2 to USB3?