With the amp in standby mode, the supply will only be going through to the cathode heaters so you’d’ve had a nice warm amp to come back to but you won’t have worn out the tubes or anything like that.

Incidentally, I’m in the camp that prefers to switch off without going through standby. If possible, I like to play a chord or note (and get that deteriorating, fuzzy sound) to bleed off the filter capacitors. You don’t end up accidentally leaving the amp on standby all night then, but it is important to remember to flick it to standby either after switch off or check before turning on.

It stands to reason that it’s ALWAYS a good idea to use the standby switch before powering up. Unfortunately, three of my amps don’t have one, and the price paid is shorter valve life. I made a habit, with such amps, of switching on with all volumes (amp and guitar) at zero for a few minutes before playing anything.

All of this applies only to valve amps. If you’re wondering why your transistor amp hasn’t got this “standby” switch; worry not.

I’m not an expert but I honestly think that there wouldn’t be much harm in what you’ve done.