VERY LOUD pop switching from CLN - BE

My BE100 makes a VERY VERY LOUD pop when switching back and forth from CLN and BE channels. This happens whether I use the foot switch, or the switch on the amp's face. It does NOT make this pop when switching back and forth from BE and HBE mode.
The volume of this pop is startlingly loud and will definitely ruin my speakers.
-Its not a static electricity thing, and I've tried cycling back/forth between channels while in standby.
-I shuffled both preamp & power amp tubes but not replaced them.

Can anyone help?

My BE does the same thing when I hold a chord or note and then switch. I don't get the pop when muting strings and switching.
Always wondered if it was the footswitch. Anyone else...?

I just tested the power tubes and 2 of them were bad.
That very loud pop was the sound of high voltage going to my speaker cab.
I'm going to see if a new set of tubes solves the problem.