As you may have seen elsewhere, the rule of thumb is generally 10x less output impedance for your preamp than the input impedance of your amp. Across the whole audible spectrum. I used the very very low output impedance Musical Fidelity kW preamp with great success with the F3. You might also want to check the 6moons review for other ideas of what might work.
Preamp help for First Watt F3
I'm thinking about getting the First Watt F3 to power my Zu Audio Druid MKIVs, but I've hear its a tougher amp to pair since its input impedance is 10k. I was wondering if you guys had some recommendations about which preamps go well with it? I was thinking about the Red Wine Audio Isabellina but not sure if that would work. My requirements are pretty simple, just a pre with the usual set of digital inputs. USB would be nice but not required.