On buying a used amp
Sometimes, particularly with relatively new gear, you'll find amps on the used market because people buy them without giving enough consideration to whether the amp's right for them, then quickly decide they'd rather have something else. So take your time deciding.
That said, here are the things I think you should do when auditioning a used tube amp. Figure for a three channel amp that it'll take you about a half hour to go through all this if you already know the amp well from having lived with another of the same kind for several months to a year. If this is your first experience with an unknown amp, spend at least an hour with it.
First, warm up the amp. Listen for hissing or crackling noises when you take it off standby. It should be quiet.
Without a guitar plugged in, turn every control and listen for scratching and pops. All the controls should be quiet. Any noise here may indicate aging capacitors or dirty pots.
If it's a high gain amp, crank the gain without a guitar plugged in. Now tap the chassis and listen for microphonics, hum and hiss (expect some at extreme gain and volume, but it should be relatively quiet) or howling (you should never get this at any useable setting). If the amp has reverb, crank it up (again without a guitar plugged in) and listen for hum. If there's any at all, it should be very quiet.
Take a look at the output tubes. Use your hand to sheild them from room light. A blue glow is OK. But if you see a dull cherry red coloration anywhere on the plate (the largest metal piece closest to the glass), turn the amp off - it's biased too hot.
Now that you've done all the quiet tests, plug in a guitar and play. Start out quietly, because you'll destroy your ears' sensitivity to nuances if you play loud. (Don't worry, we'll get to the part where you crank it.)
Turn the controls to a lot of different settings and make sure that the amp stays "musical" over a wide range of tones. Your tastes may broaden or change, so it would be nice to find an amp that can grow with you.
If you're looking at a channel switching amp, try all the channels. Make sure you can get good sounds out of all the channels.
If the amp has option switches (like bright, fat, shift, ...) try them all and make sure they actually work. If the amp has a footswitch, make sure it works. Don't assume anything. It's better for you if you can identify a defect before you put your money down - you can use it as leverage to get a lower price. If you discover a problem after you get the amp home, it's money out of your pocket to get it repaired.
If you get this far, bring the amp up to performance volume and play. Listen for buzzes, rattles and undesirable distortions. Right after you finish playing at volume, quickly take another look at the output tubes and make sure that you don't see any cherry red plates.
Finally, get your nose right up to the amp and sniff. You shouldn't detect anything that smells like it's burning.