My beloved Mesa Boogie Mark V:25 started humming quite perceptably recently and I'm looking for ideas on what to try before I call Mesa, box it up, and ship it off to them (under warranty, nearest service center is a 3.5+ hour drive away). Totally bummed because it has been my #1 since I got it and it had a very low noise floor. This hum is loud enough to be heard when playing. Symptoms/observations: *It is a lower frequency hum, not a buzz, crackle, etc. *Hum is present with power on as soon as I flip the Stand By switch. *Hum gets louder as I roll the Master Volume up on either channel. *Hum is present in all six modes; clean, fat, crunch, IIC+, IV, and Xtreme. *Hum is louder in 25 Watt Mode than in 10 Watt Mode but clearly present in both. There is now a noticeable "pop" when switching power modes (this is new...). *Hum is present regardless of EQ on/off or settings. *Hum is present and unchanged regardless of whether or not an instrument is connected and no matter which cable or instrument is used (all sound fine through my other amps as well). *Hum is present and unchanged regardless of Reverb level. *Hum is present when connected to other circuits in the house, with and without the conditioner. *Hum is present with or without the Footswitch connected and regardless of the position of the associated switches. *Hum is present through another 8 Ohm cabinet. *With Speaker Off (switch) and headphones plugged the hum IS NOT present! ...dead silent, sounds like it used to through the cabinet. This should probably tell me something but I don't know what it means! Nothing is in the loop and no pedals out front. All other amps (Fender SFVR & Marshall Class 5) have a very low noise floor on the same circuit through the same conditioner. Power tubes look normal during startup and operation. Taps on preamp tubes with a pencil don't produce anything noteworthy. I've ordered a new duet of power tubes and a couple of preamp tubes as I probably should have a spare on hand anyway so I can give them a try in a few days...but, because of the headphone thing, I don't think this is the issue. What else can I try? What does the fact that it's silent through the headphones but hums through a speaker cabinet suggest?