Sean, great answer, as always! There are even some pre-amps that will produce an audible "thunk" through the speakers when switches are thrown. Also, the louder you play your music, the less isolation that you provide for your preamp, the higher the gain of your tube circuit, then the more problems that you will have with good tubes being microphonic.
There are some tubes (small signal tubes, usually NOT power outputs like the EL-34's) that are susceptible to being microphonic. There are some rather unscrupulous dealers [no names!] who will not check for badly microphonic tubes, which will test out just fine on their tube testers. I highly recommend Andy at Vintage Tube Services...he's the best, and has helped me to undo the mistakes of another well known tube guy! http://www.vintagetubeservices.com/
There are some tubes (small signal tubes, usually NOT power outputs like the EL-34's) that are susceptible to being microphonic. There are some rather unscrupulous dealers [no names!] who will not check for badly microphonic tubes, which will test out just fine on their tube testers. I highly recommend Andy at Vintage Tube Services...he's the best, and has helped me to undo the mistakes of another well known tube guy! http://www.vintagetubeservices.com/