Make sure it is dead quiet. There should be no buzzing coming from the amp or the speakers. If there is buzzing, don't buy it, there is something wrong! Anybody that tells you that you are supposed to get buzzing or hums from a tube amp has only listened to "broken" or poorly designed tube amps. There is no way an amp can convey good sound, an accurate sound stage and well reproduced music even if the music has higher volume than the background noise (noticeable buzz or humming from the speakers). If it is dead quiet when you listen to it someplace else, it certainly won't be when you get it home. Never buy a used tube amp (or any amp for that matter, especially on-line) without requiring the selling to state that the amp is dead quiet both at the amp and through the speakers (with speakers that are at least 88 db(A) efficient). If an amp seller states that some buzzing is part of the tube amp protocol, run like the wind for that purchase.
Auditioning a Tube Amp
I've never heard tubes and am seeking my first purchase. I'm going to be buying local and auditioning. What should I be looking and listening for to ensure everything is on the up and up? I'm leaning towards something newer like the Sophia Baby so as to not end up with a "project." Any specific tips appreciated.