I'm running an ART DIO modified to a smART DIO by Boulder Cable co.
its breaking in...and sounding great, a huge upgrade from my Philips 963sa player...which was yards better than any other CD player I'd heard before (upsamples).
there are two upgrades for the ART, the smART and the MENSA. You can get info on boulder cables site here:
http://store.wellsfargoestore.com/Store.asp?m=TheBolderCableCompany&n=10&k=79313&s=ART+DI%2FO+Accessoriesdo a little research on this (as I have), and you'll find people comparring it to real high-end DACs that are in the many thousand dollar price range.
another inexpensive option that I have read a lot of great things about is the Ack! dAck. google on that as well. I think it goes for $5-600 or there abouts.
Back to the Benchmark. I've heard Carls, and LOVE it. Yes, it sounded better than my DIO, but not overly apparrent on all material, and it was nowhere near broken in when we did our listening tests.
Its ability to act as preamp (by adding gain) is wonderful for a digital system. No remote though.
I also hear lots of good things about the Scott Nixon tube DAC. Also under a grand I believe.