When I do something like this I use a looper pedal or reamp, so i can record the same perfomance twice - eg before and after a (tube) change. Get a 'good' perfomance with different playing styles - hanging notes, chords, palm mutes etc. Of course everything else (mic placement, volume etc.) must be the same.
ISOL8 is a great plugin to check (with your ears) which frequencies is affected
Get it here for free
http://www.tb-software.com/TBProAudio/download.html. I prefer to have it on the 'monitor effects' in reaper.
-W