Why's async USB so important?
Find a used Benchmark DAC1 USB for under $1000 (this is a rather sterile and cold sound, though, transparent - not warmly colored as you might prefer), it's got some of the best adaptive usb implementation around. It is not as if the worst of the async's are better than the best of the adaptives; I do not believe the differences in sound between implementations is a gulf so great as that.
Anyway, I'd say don't worry too much about adaptive vs asynchronous, and go for as transparent and distortionless a DAC as possible. Let the amplifier and the speakers or headphones provide your color.
You might also refer to the lists found here: http://thewelltemperedcomputer.com/HW/USB_DAC.htm to get some ideas of models that are out there.
Edited by Timestretch - 1/11/12 at 10:13am