Hi Monty,
I forgot who it was, but someone on TGP called Mesa and they did, indeed, confirm that this "muting" when channel switching is designed-in ...yessir, on purpose (this is WRT the Express models).
OK, that said, I have never noticed it on my old Marks, or on my current DC5 --which has been my live go-to amp for clear over a decade. YET, on my 5:25, I do hear a very short "muting" when channel switching. It is imperceptible if simply switching channels between songs. But if switching on the fly mid-song, then yes, my 525 absolutely has it; and none of my other boogies past/current does. It's a bit disconcerting, I agree, but only when switching on the fly mid-song, and ever so short; therefore not a deal-breaker for me. It just means I have to work around this idiosyncracy. Were the amp not "that good" in everything it does for me, I'd have sold it long ago. Only because it sounds/work so well for me, I live with it. A bit unfortunate, yes; but given its attributes in excellent tone and superb dynamics, I'll accept this annoyance.
Now I would think that this design would manifest in every Express amp. That you didn't hear it on one model in a store makes me wonder if perhaps you merely didn't notice it (other store noise/players, etc), or that each amp varies perhaps in the length of that mute? Who knows....
Edward