Well, I opened up my 400+, tightened all of the tubes, and ran it for a while. There was one tube that seemed to light up much brighter. Is this something that I need to replace immediately? The amp did a three hour gig with no problem at a pretty moderate volume. (preamp 6 and master around 6) Also, I have the boogie 430 tubes installed and these have less than 40 hours on them. Boogie no longer makes them. Can I replace the bad tube and its correspondant tube with a matched 6l6 pair?