This is a source of on-going astonishment for me, the microphonic 12B4. I have a small stash of them and have not had any more bad ones than any other sort of valve. Perhaps the sensitive 6SN7 crowd pushed this a little askew, but I have not found the 12B4 particularily microphonic. Two 12BY7's per channel make the active-load/output buffer. 12GN7 is a direct plug-in and at 250% the gm, it won't be ong before I put them in.
Still need to get a few bits to put her together. cross your fingers.
regards,
Douglas