I had a horrible hum on my Checkpoint. I replaced C4 (the 18000uF cap) with 4x4700uF caps and the hum is pretty much gone. I have since picked up a TFTC that hums pretty bad. The power supply on my TFTC has the 4x4700uF caps installed so I'll be replacing them with hopes of reducing/eliminating the hum as well.

I'd probably replace all the caps for all 3 outputs and the power section (assuming you determine it's the sound board). I get all my stuff from digikey. Caps are cheap and easy to de-solder so I figure look at the schematics and start replacing ones that seem to be the most likely culprits. You might want to consider replacing the C4 cap on the power supply as the one on my Checkpoint was beyond dead. Make sure to get the higher temperature caps on the power supply!!

interesting... could you have unintentionally nudged a cable while you were replacing those bulbs that might have solved a "ground loop"? What happens when you put the old bulbs back in?