> My requirements are pretty simple, just a pre with the usual
> set of digital inputs. USB would be nice but not required.
Most 2-channel preamps do not also function as a DAC.
Having said that, I would recommend the Modwright LS-100, which does not yet have an option for a built-in DAC, but is expected to within the next few months. Of course, you could always just get the DAC separately. So many choices for DACs these days, I'd probably decouple the buying decision.
The LS-100 has an output impedance of 300 ohms, which should be plenty low enough for the 10K input Z First Watt F3. I've had a few First Watt amplifiers through here, and several Modwright components, although funnily enough, not at the same time. Nevertheless, I think this would be a good pairing based on overall impressions of the two manufacturers.
Another option may be to get a DAC which is optimized to not require a preamp. If you have no analog input requirements for your preamp, perhaps the NAD M51 or the PS Audio Perfect Wave DAC MK2 would do the trick for you.