I've read many different articles on the subject about replacing the 6L6 tubes with KT66 tubes and obviously everybody has an opinion. The guy at the Guitar shop that sell Mesa amps and also told me has has an older 5 50, which is what I have as well says he has a set of unmatched KT66 in his amp at home and has had NO problems with it, anybody else done this ? or is he just trying to sell me some tubes ?