You might buy some new production 6SN7 tubes...like Shuguang...and install those. The new tubes should not be microphonic, especially if they are purchased from a dealer that screens for this problem.
If the preamp has microphonics with the new production tubes, then you'll know you have a problem with the preamp and not the tubes.
If the preamp has microphonics with the new production tubes, then you'll know you have a problem with the preamp and not the tubes.