I have a 3 Channel Triple rec that I bought new in March. It 'Popped" on intial switching from day one.
I called Mesa and they explained that this normal static discharge after powerup. They said when they designed the amp they addressed the problem and they could have eliminated it, but it would involve a built in modification to the amp that would create a delay each time when you switch channels to allow for that discharge. Since it generally ONLY is supposed to happen on initial powerup, if you simply discharge it on powerup, then there shouldn't be a problem during normal playing.
On my amp it does NOT matter whether the amp is turned up or all the way down.... the 'POP' sound is always the same volume thru the speakers. AFTER I have come out of standby I need to switch through all the channels a couple times. There is usually 2 pops one medium, one loud. Sometimes the first pop is right away.... other times it is the second or third switch... that is why I switch thru several times to get the static fully discharged. After the big 'POP', it switched fairly quietly and has NEVER been a problem.
Let me add... that sometimes in a hurry I only switch once or twice, and get a pop, but there is still a static buildup and later while playing I go to switch and it pops. That is why it is important to switch multiple times thru ALL channels several time AFTER coming out of standby. Switching in standby without a output load doesn't work on my amp and the static won't discharge. I have to be OUT of standby.
I wasn't real happy with this, but I just make it part of my regular routine just like warming up the tubes.... and it really is no big deal for me.