In what might be one of the more expensive USB audio solutions for the mini, it turns out several Pioneer Elite A/V receivers accept digital audio in over USB. The models I could confirm that accept USB are the, VSX-56TXi and the VSX-59TXi.
The 59TXi lists for gulp $4500.00… not exactly a bargain basement solution, but the VSX-56TXi can be found online for around $950.00, again not for those on a budget, but if you were already in the market for an upper end A/V receiver, for use in a system with a Mac mini, this makes the deal a little sweeter.
The VSX-56TXi is rated at 110 watts, supports all the current surround sound formats, comes with a rather nice universal remote and one ultra-geeky feature sure to please your inner tweaker, Pioneers MCACC (Multi-Channel Acoustic Calibration) which will “listen” to your room and compensate for room deficiencies such as bass nulls, over absorbent room finishing’s and even plot out your rooms sonic curves in real time. Something to keep in mind if your current receiver is about to give up the ghost.
Alright scratch this one off the recommended mini USB audio list for the time being. conny posted a comment that he has one and the receiver will only recognize PCM stereo.