I have both the Benchmark DAC1-USB (not PRE - but the DAC and headphone amps are the same) and a Headroom "maxed-out" Desktop amp/dac (now known as the Ultra Desktop Amp -- they are exactly the same). Both of these amp/DACs sound good but do have their differences. The Benchmark's strength is mainly its DAC, which is really quite good and very resolving in general, although some have found it to a bit dry and forward-sounding, myself included. The headphone amp section unfortunately does nothing to mitigate the rather forward sound - perhaps it was designed that way - however on some phones, it works well. I can say that I did not like the sound of my AKG 701s with the DAC1 but it was pretty good with a cheaper pr of Grado SR60s. The Ultra Desktop, I feel, is probably the overall better performer -- it seems to do more things well compared to the DAC1, however I do not think the Ultra Desktop's built-in DAC is quite at the same level as the DAC1 in terms of ultimate resolving power. The AKG701s certainly sound better with the HR amp compared to the DAC1. Another DAC/amp that many folks seem to like well is the Grace M902, which is also priced about the same or maybe slightly more.
BTW, if you're leaning toward Headroom products, the new UDAC/BUDA combination (which I also have) sounds better by far than either the Benchmark or "regular" Ultra Desktop, and gives you the opportunity to go balanced, if you think you might go this route in the future. The downside is that the combination costs about $1000 more and you have two boxes instead of one (plus the DPS).