I called a local amp tech recently and asked if he could install new tubes in my son's Marshall JCM-900. I asked him which tubes he recommended and he said it didn't matter because, "tubes are just 5 pins and a vacuum...how can they affect the sound?" He believes all of this talk about tweaking amps by swapping tubes is a rip-off. He has been working on amps for decades and is well respected. For the record, the discussion was focused on power tubes for this particular amp and I mentioned swapping JJ KT77's for the currently installed tubes because some "metal players" (like my son) seem to prefer them. Not sure if his comments were meant to imply that this particular type of tube is not worth trying or if the whole concept of tube swapping is over hyped. I didn't press him further because he became a bit grumpy at this point. So, is this tech right or is he wanting me to sell me his chice of tubes? Note: He did not mention any particular tube and his prices have always been very reasonable in the past. Just trying to figure out if this guy's advice is trustworthy. Thanks for any help on this. I hope this doesn't start a heated debate--not my intention. I have no opinion of my own because I honestly don't know.