I play guitar in a Rockabilly trio (guitar, electric bass, drums). The drummer uses electronic drums through his own system and we have a small PA which is perfect for vocals, but totally inadequate for anything else. We play at a low volume and don't mic anything other than vocals. We played a gig last week and everything was working perfectly. Last night, we get everything set up and the bass player plugs in, no sound. Check all the settings/switches, still no sound. Swap out all the cables, still no sound. Fortunately, the guitar player (me!) ALWAYS brings a backup head (Ampeg PF-800). We plug it in and are up and running in 5 minutes, start on time. If you think bringing spares is overkill, you're just wrong. The head weighs about 12 pounds, takes up very little room and is adequate for bass and guitar. This is one of our regular clubs, and although we could have run him direct through the PA with the vocals, it would have been really ugly.