Peavey classic 30 (like most other amps) isn't class A.Its just push-pull parallel ab1/ab2, most likley cathode biased, and biased on the hot side.
That aside class of operation is really of no relevance to an amp needing a standby switch.Stand by switch is really only useful or two things, A allow the tubes to warm up a little before you actually apply voltage to the plates and screens B if you gig a lot, and want to leave the amp at its set level without actually having the amp running fully between breaks etc,but if it was me I would just turn the bloody thing of.
On amps without stand by switches if you just power them up with all volume controls on zero and don't provide it a input signal and give the tubes a couple of minutes to come up to temp the amp will be fine.To turn of ANY tube amp with or without standby switch you should be switching the amp straight of, NOT to standby first as is commonly thought.
If you must have a stand-by switch, it is by all means a cheap and easy mod, just not really needed.