Even so, you'll still find that it's best to have matched power tubes, since most amps give you a single bias point for both/all power tubes, so if the tubes aren't closely matched, the bias setting can only get you somewhere "in-between" them (though some amps give you separate bias for each tube, so you can use unmatched tubes too).
I set my own bias (I have the tools) on all my amps, and I've messed with a variety of settings on some amps, going from their minimum to maximum desired points (and even a beyond)...and I've never had any make that noise when powering up even with the bias way off the mark and with unmatched tubes.
Bad bias settings would show up more during playing...I would think...not so much during the power-up cycle. Anyone have opinions on that?
Does the amp have a Standby switch?
If so, does it make the noise during Standby or only when power ON is applied?