I have a 1970 Twin Reverb that developed an issue quite a few years ago. A tech diagnosed it as a failing power tube, so I removed the inner pair of 6L6's and have been using as a two power tube amp since then. I've decided to restore it to a four power tube amp, and plan to order a replacement for the bad tube. However, I neglected to mark which is the bad tube. Can I simply replace one of the good pair that are in the amp with the tubes I took out one at a time to find out which one is defective? I've heard that bad power tubes can take out the transformer, and don't want to put the amp at risk. I don't have access to a tube tester, so if there's a danger in trial-testing to find the bad tube, I'll order a matched pair.