I'm also in the market for a <$1,000 DAC and I too am looking for advice.
X-DAC? - Great reviews, Stereophile recommended, I have X-PSU already, price is right. no 24/96 or USB
Benchmark? - Big support in this forum, can be used as a pre amp, no usb, occasionally see them used.
Stello DA-100? I really like this one but don't see them used and I'm little scared ordering something from overseas. If I could find one stateside I'd likely buy it. No reviews, no one has heard it yet. I listened to it at CES but only for a few minutes.
Bel Canto DAC-2? Good feedback, no usb, often seen used
Grace M-902? $1,500 (ouch), has usb, selectable sampling rates, headphone out, pre-amp capability, great reveiws on head-fi. If I could find one for $1,000, I'd buy it today.
Scott Nixon Tube DAC- Who knows how it sounds, tubes are cool.
Do I care about the difference between up/over sampling and NOS non-over/up sampling DACs?
Do I care if it is a Delta/Sigma or not?
Primary use - Headphone system
Souces - Marantz transport, I-Pod (apple lossless), Music Hall MF-5
Phones - Senn 650's
Music - Frank Zappa, Jacques Loussier, AC/DC, Bella Fleck,