With my 3-channel non-multiwatt dual recto, the pop when switching channels can be mostly alleviated by cycling through the channels with the footswitch when in standby after turning the amp on.
Also when I changed power tubes to a brand new set of the STR440’s, the popping was diminished a lot, not sure why. With the old tubes (I believe original, small-bottle STR420’s) it popped very loudly the first time I switched channels, then it was fine unless I was sat on the same channel for a long while before switching again.
I very much like channel 2, but I’m not a big fan of channel 3, I don’t use it much at all. The FX loop also sucks in my opinion.