Can using various dampening rings such as Herbie's reduce or eliminate most microphonics? Or is the microphonic the result of internal parts vibrations?
IME, Herbies can reduce microphonics up to a point. Once a tube has gone full-on microphonic nothing can help. Tube dampers lightly touch the glass envelope and reduce resonances, but in many cases the sound will be affected. It may make the bass tighter in a good way or it may reduce the liveliness of said tube, making it sound dull.