[jseth]

My '75 Deluxe Reverb does the same thing id the volume is up and the reverb is on... basically, it's the power surge, going from 0-60 when you flip that switch, would be my best guess. I've owned a few old Fender amps and played through a BUNCH of them, and all the ones that had the standby switch do the same thing.

Turn down the main volume for the channel you are using and dis-engage the reverb; that will quiet it down a lot.

There is much discussion about the "right way" to both warm up an old Fender tube amp and to turn it off... one school of thought is to flip BOTH switches UP when you turn it on (but have the volume on your guitar/channel down), let the tubes warm up for a bit (5-10 minutes) and you're good to go... then hit both switches DOWN simultaneously when you turn the amp off.

I use the stand-by for the warm-up period, but hit both switches when I turn it off...

I think the most important things are to let those tubes warm up before you start playing, and to be sure not to move or jostle the amp when you first turn it off, letting the tubes cool before they get moved around...

A Twin, eh? Well, you must have a MUCH stronger back than I have! They are great amps; haven't tried one of the re-issues yet, but if they're even close to the "real deal", they must be very good and VERY loud!