If you want to start minimally, damping the input tubes would be a good place to start. I suggest starting with all the small-signal tube, though: input, pre-driver, and DC restoration. These tubes use inexpensive UltraSonic SS Damping Instruments, so the investment might not be prohibitive.
It's impossible to say which of these tubes would benefit most from damping instruments. All have the same potential for microphonics and micro-vibrational distortion, the degree of which will vary from one tube to another and from one component to another. I'm not sure of the circuitry design of the 180, but the DC restoration tubes might be the most essential to damp. Like with rectifier tubes in many amp circuits, the DC current they provide affects other tubes, sometimes all the other tubes in the component. You don't want a fuzzy DC current feeding the musical amplification.
Each stage of the vacuum tube circuitry works hand-in-hand with the next stage. Damping one stage will send a more faithful rendering of the music to the next, which can subsequently add microphonics right back onto the musical signal. Conversely, with an undamped, distorted signal going from one stage to the next, that distortion is then part of the music itself and amplified along with the music.
Damping the small-signal tubes in your amp will most likely bring out a significant improvement in the amps' musical potential. From there, damping the power tubes would be icing on the cake, if you're inclined to go a step further. KT88s are generally receptive to substantial improvement with damping instruments (either UltraSonic Rx-50 or HAL-O III-50). With a dozen tubes, that can be quite an investment, though worthwhile to bring out the full potential of the amps.
We've found, however, that damping just half of the power tubes in a multi-power tube amp usually provides substantially more than "half" of the improvement that you'd get by damping all the power tubes. Damping just every-other power tube instead of every one of them is recommended for the budget-conscious. (With monos, apply the damping instruments in matched stereo pairs.) You would probably find that damping half the power tubes is more than sufficient.
Herbie's Audio Lab