I also read somewhere that VT-231/GTs were prone to microphonics compared with the GTA/Bs.That's certainly been true in my MP-1. I've tried two pairs of Tung Sol VT231 round plates and two pairs of Ken Rad VT231 black glass, and 5 out of the eight tubes have been microphonic in the front position of the MP-1.
Pubul57
When non-microphonic versions are discovered, they may offer excellent sound, but I'm quickly concluding that for me the hassle of finding them (not to mention their cost) is outweighing the slight benefits.

