I currently have the Van Alstine Vision Hybrid DAC, which I like very much. It is connected to my Mac right now via the KingRex UC 192 converter - which seems a very competent converter.
I have not tried an asynchronous USB converter but have tried a few asynchronous DACs. I've tried a lot of DAC's. I have not found the asynch ones to automatically be better than non-asynch ones. Its the total implementation baby not the the individual technologies.
Thanks for all the info, everyone. I am thrilled that I would not have to convert all my ALAC files. I am willing to work through the cover art issue.