chungjh,
1) I don’t know why each of the tube’s blew. With the JL3’s & their internal meters, they were easy to spot however. Having new sets of power tubes in both the JL1’s & JL3’s when I got them, I never changed the entire complement of tubes, because of a tube failure. So, the tubes that failed weren’t old per se. The fact remains that in over approx. 20+ years, only 3-power tubes developed a problem & 3-resistors were needed to be replaced. In general, then, power tubes were not a concern during my ownership.
2) I used a new set of 6550’s in the JL1’s when I got them and began with 6550’s in the JL3’s which my pair originally utilized. My CAT dealer retrofitted the JL3’s to use KT120’s & sourced the new tubes from CAT which submitted them to an in-house stress testing procedure. The retrofit required a change of the sacraficial resistor (as one would imagine). But more than that, I am not entirely certain what else was changed, if anything.
3) The JL3 with twice the number of power tubes than the JL1, was obviously more powerful. However, I never actually A/B’ed one amp against the other in a back-to-back test. But my audio memory suggests that the JL3’s were NOT sonically different, or superior to the JL1’s.
Around 2008, I had an opportunity to trade the JL1’s for a preowned pair of JL3’s with my local CAT dealer. It was a deal I couldn’t refuse, but not one I really needed!
With my old non-PX panel, non-consummate transformer M1 SoundLabs, a handful of tracks (no more) seemed to benefit from the more powerful JL3’s. This was perhaps because the JL1’s clipped slightly on a passage in those tracks). It was never a real bother however. But with the newer SoundLab panels and transformers, I would assume that the power difference between the 2-amps would NOT be noticeable due to the power increase. Sonically, there may be a difference, but not one I believe would justify the cost difference between the 2-amps. When I auditioned Atma-sphere MA1’s against the more powerful MA2’s at a SoundLab dealer on U1PX’s, there was a difference that I termed gravitas-like (e.g. perhaps a the MA2’s were a little more dynamic). Could the difference have been volume related between the 1st and 2nd trial, yup, we didn’t use a decibel meter to check. But as I suggested for the JL1’s & JL3’s, the difference was too subtle and not worth the extra expense, either upfront and later, if re-tubing were necessary. -Mike