I was running sound a few weeks ago and came across a situation where the guitar & bass amps were not miked and the vocalist was not singing loud enough. Yes, the guitarist's amp was too loud and was drowning out the other musicians in the band. Basically, it was an impossible thing to fix at the mixing board for the most part.  I was able to tweak the vocalist's channel but I was at the point of the PA feeding back... Here is the tip...  Before you perform your show, how ever long that may be, make sure that everyone's volume is balanced. Stand out about 20 or 30 feet from the stage and let everyone play a brief piece of music. "Please, very brief".  You will get a better idea of what the audience is hearing. Remember, the sound man cannot fix or adjust any amps that are not going through the PA. If you don't help the sound man out, he or she can't make you sound awesome...  Yup, pretty much...