What we finally did to avoid the hum during recording is buy a USB Dual Pre (preamplifier) which connects our studio mic directly to the computer. No hum. Cleaner sound and easier setup for sure although it sometimes acts funny with my sound card, cutting out and needing to have the devices reset when I unplug anything to listen via computer speakers rather than headphones, etc.

Stu, we did get some occasional noise (although no hum) even with this preamp, and your list is useful for us to check. I thought it might have been something electrical running in the room or elsewhere in the house, and you've given me several ideas to check.

I'm trying to get the hum out of previous recordings because the artist lives several states away and is unlikely to be able to re-record with us. More about these precious recordings when/if they are released.