You'll need a known good tube. Swap it in, one tube at a time. If you hear an improvement, then you know that known good tube is helping.
But you may have more than one bad tube- they have a way of going slowly downhill; at some point they can get so weak that the performance of the circuit reaches a tipping point. So you might want to try swapping out more than one tube with a known good one.
But you may have more than one bad tube- they have a way of going slowly downhill; at some point they can get so weak that the performance of the circuit reaches a tipping point. So you might want to try swapping out more than one tube with a known good one.