porscheracer - "Anyway, I am pretty sure the hum is gain related. My preamp has 14dB of gain in tube mode and the amp has 27dB. That's a lot of gain for speakers as efficient as the Double Impact. If I put the preamp into passive or JFET mode with 0dB gain, the hum gets quieter but it is still there. If I unplug the RCA cables from the amp, it's gets a tiny bit quieter. This mirrors what the preamp and amp designer said would happen with high efficiency speakers.The First Watt F7 has 14dB of gain so I would be cutting 13dB. About the same as going into passive or JFET mode on the preamp."
Not sure if you got your excessive gain/hum problem resolved but if not try out the Rothwell attenuators. They are plug and play a come in -10db, -15db and -20db. I've used them in various systems over the years with great results. Here is a link: http://www.rothwellaudioproducts.co.uk/html/attenuators.html