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I'd check out and talk with Larry Moore of Ultra Fi regarding his iRoc USB DAC.
The iRoc is very reasonably priced and Larry has one up for auction (a promotion) on Audiogon right now. He also has a very generous (imho) return policy.
I really like the results of the iRoc DACs I own, both on tube and ss.
iRocs have been used by other mfgrs at CES to promote other products.
I don't think you will be disappointed, especially if you use a MacMini with high quality USB cables (synergistic research or ridge street audio) and ICs from the DAC to your preamp.
:) listening,
Ed
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