I've owned an SP-3a (upgraded to "c") since 1974. An LS-15 since 1999, a D76 (from 1975 to 1980), and a VT100 MkII since 2002. No problems with the pre-amps. No problem with the D76. Only problem with the VT100 is one of the output tubes died, which I could determine when I checked the bias and it was very low in one channel. I could see that one of the tubes was not glowing. No other damage was done. I then replaced all the output tubes, re-biased the amp, and it's been fine ever since.
PMOTZ ; I have heard just the opposite about tube layout. An experienced electronics tech I know says that with vertical output tubes, as the tubes age, metal flakes can come loose and find their way down to the bottom of the tube, where things can get shorted. This is less likely to happen with horizontal tube layout.