JTM45 because I can get beautiful full singing cleans, or great overdrive to see if the guitar (and its pickups) hold together well. Some will lose all note seperation rather quickly when pushed, plus I can see if there is too much inherent feedback in the guitar/pickup combination.
None. Study the sound of the guitar acoustically first. If it doesn't sound great unplugged, you'll just end up fighting it later- swapping pickups, amps, etc....