Some amps have lots of output power tubes. A pair of Cary V12 monoblocks uses 24 output tubes, each prone to microphonic distortion to some degree. A pair of Atma-Sphere MA-1 monos uses twenty-eight 6AS7 tubes, a pair of MA-2 monos uses forty. We have one customer with eighty-four 6AS7 output tubes in his audio system. Another uses an UltraSonic Damping Instrument on each of two hundred and forty KT-88 tubes in his home theater!
The cost of damping microphonic vibrations in so many tubes can do quite a job on the bank account. The investment is worthwhile, considering that damping instruments will bring out more of the full potential of the much larger cost of the tube gear itself.
With a limited budget or frugal approach, it always makes sense to damp source and small-signal input/driver tubes first, and most importantly, any rectifier tubes. It needn't necessarily break the bank, however, to damp the power tubes also and reap the additional sonic rewards.
I have found, as have many of our customers, that damping every other stereo pair of output tubes often achieves a sufficient and desirable level of damping. Keep in mind, properly designed damping instruments damp vibrations and subsequent distortion--not the music itself. What we've found is that with amplifiers having many output tubes, damping half of them seems to achieve substantially more than 50% of the benefit you would get by damping all of the power tubes.
1/2 = more than 50%. This is a purely subjective analysis, but when it comes to listening and enjoying music, the subjectivity--how well you enjoy the music--is all that really matters. Damping half of the power tubes by placing damping instruments on one stereo pair and then skipping one pair, etc., allows the inherent "flavor" of the tubes to come through unfettered, yet keeps microphonics from muffing things up.
Of course, if you should try this and feel you want to go even further in the same sonic direction, there's nothing wrong with damping all the power tubes. As a matter of frugality though, if you have eight or more power tubes, you might try damping just every other pair--you'll likely find that to be sufficient and completely enjoyable.
Steve HerbelinHerbie's Audio Lab