The Ayre QB-9 DAC is remarkable. I've had mine for about a month, and it revolutionized my system. I can't say enough about it; I often feel like I'm hearing my music collection for the first time. See the rave review in last month's Stereophile (now online) for more info.
Having seen it demonstrated in a couple of ways, I am now a firm believer that jitter reduction as an absolutely critical element for digital sources. The only USB DACs I would consider right now would be the Wavelengths and the Ayre (because of the async USB), and I wouldn't go near SPDIF unless it was reclocked with something like the Empirical Pace Car. Or maybe use USB with Empirical's Off Ramp into one of the DACs the others here are recommending. Though, by the time you do that, price wise, you might as well just get the Ayre.
Good luck,
Bruce
