I can't give a specific db level or anything (my caps are in desperate need of replacement), but it sounds like your caps are at least starting to fail, as you shouldn't have to have the volume at full tilt for it to sound 'normal'. Most problem GTs and Expresses seem to be inaudible until they are turned up at least half way and have to be cranked to be heard comfortably, and eventually get so bad that they're barely audible even at full volume. At least it's a fairly easy fix, though you could sell it as is and just describe the volume in the listing; I doubt that it'd make much difference in the end value.