It's been a few days, so I'd like to partially close the loop on this issue. A few days ago, there was no hum when I turned my system on. This isn't unusual as the hum comes and goes with no discernible pattern. I also installed beefier interconnects between my amplifier and preamplifier. To address an issue raised in an earlier reply, I only have my computer hooked up to my system. I don't have any other components right now. The computer is attached via TOSLink to my preamp. One night when my system was humming, I shut my computer down and unplugged it from the wall. The humming persisted, so I surmised my computer was likely not the cause of the hum issue I was experiencing. I haven't heard anything about TOSLink being a source of hum either, so I ruled that out as a likely cause.
Fast forward to today. I'm on Day 4 with no hum. The absence of hum may simply be due to a neighbor who went on vacation or my issue may truly be solved. I won't know until more time has passed.
Thank all of you who replied. I'll certainly keep your replies in mind should the hum reassert itself.