gain control

I have 2 different MarkBass combo amps. I took one of them into a MarkBass service shop yesterday. The service rep said that I had the Gain and Master settings wrong. He stated that I should have the Master turned all the way up and bring up the Gain as needed. This is completely contrary to...

I Purchased a used Fender Rumble 500 combo knowing that the gain control was not working correctly. When it is turned to full volume it works fine. All the rest of the range does not work at all. I am gigging with it with the gain all the way up. Should I replace the pot? I am pretty handy and...