If you're turning the bias pots to the extreme positions you could be damaging the tubes by that very act. You need to find out what the correct bias setting is, then get yourself a multi-meter and set it to the correct setting for each channel. If that can be achieved and the amp plays fine you're probably okay. If it won't bias correctly a tube (or tubes) could be bad. On many amps a fraction of a turn on the pot makes a big difference in the bias setting...
If you're not sure what the correct biasing procedure is, you should get the instructions for your particular amp and read them before attempting to set the tube bias.
If you're not sure what the correct biasing procedure is, you should get the instructions for your particular amp and read them before attempting to set the tube bias.