i would go with what Jake has stated. sounds like a bad driver tube. it could also be lose in its socket. i have had that happen to me a few times. if you can get at the driver tubes then take them out and put them back in. might want to try some deoxit on the pins. i never had a power tube make the sound you are hearing. not going to be your wire or PC. you would get hum but not the sound you are hearing. so start with the driver tubes.
good luck.
