One channel gets very quiet after bias is adjusted

Time for new tubes!!!!!!!!

My right channel sounded less lively and tiny bit less lound on a number of records. So I swapped the tubes left and right. It is a 300B SET amplifier and now the left channel had this kind of laziness to it. One of the tubes where laziness is observed has small burn out stripe at the very top.

You are routing most of the current to one side when you do that,so sure,one side will be louder than the other.Why do you feel the need to do this anyway?Just curious.

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.