Anyway. All of those little knobs are call an Equalizer or "EQ". The numbers represent different frequencies. To be honest. I dont know much about the numbers themselves, but I know about the sound that they create. Ummm...let me see if I can make this clearer and help out.
A) The equalizer and what settings you have it at isnt going to affect the performance of your amp. Its only going to adjust the sound that comes out of it. There is no specific way they should be set....just adjust them to what sounds good to you. Typically, if you want a good sound...you\'re EQ qill end up looking like a slight upside down arch, but it varies. Again, its all on about what you think is good. However...I\'ve known some amps to go haywire if you mess with the EQ because there might be something wrong...such as a short on your circuit board, etc. Thats most likely the case. You\'ll just have to try your best to set your EQ back to the way it was before. Play with it a little until you can get it to stop rumbling. That or have it fixed by a pro. Better yet, buy a new amp because it would be better use of your money. Save all of your money and buy and Orange bass amp with a 4x10. I play Orange amps for guitar and I\'ll never turn back.
B) I\'m not sure what kind of music you are playing, but I would use the high gain input. The sound is way clearer. Also, bright can be okay, but dont worry about contour and all of that shit. Thats another reason what I\'m into those amps. Try playing through the high input. The rumbling could be coming from the low input, but most likely it isnt.
C) You may have just blown a speaker? Sometimes you cant tell unless you turn your amp up really loud. But that probably would have been obvious.
All in all, I\'d go with "A". You probably have a short that lies somewhere within the EQ. My roommates bass amp was the same way. You just have to spend a little time playing around with it until you can get the rumbling to go away. I hope this helps in some way. Good luck man.
PS-That only sounded crazy to me because I\'m judging that "little man" probably doesnt play on a stage with full sound with access to run his amp direct into the board. If he is playing venues with full sound, he might want to consider picking up a new amp anyway. (Not trying to bash on your amp either man)
I have an old Laney bass amp and it does the same thing. If you adjust the eq on it, it goes nuts, but if you set it back its fine. Since he said it was working properly up until he adjusted his settings, I figured it is probably the same problem.
By the way, I never noticed you live in Boston. Have you heard of a band called Cave In?
