1 - cab/finish damage that wasn’t covered in the original listing (whether new or used)
2 - a full tube set, with the right tubes in each position
3 - all the knobs and switches are where they’re meant to be and as per listing
4 - speaker connected at both ends of the cable
5 - power it up on standby. Does it power up? (Don’t plug in guitar yet)
6 - take it off standby. Do you get sound out of the speaker (hiss/hum)?
7 - plug in guitar, can you hear it through the speaker?
8 - check all the knobs and switches do what they’re meant to (inc. footswitch, if this amp comes with one)
That’s about all you can do to check it isn’t a lemon - any other problems will show up over time - which is just what happens with tube amps.
Enjoy!
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