MTI SVT. When I tap on the preamp tubes (the ones in the top part of the chassis), all of them make a sound. Are they ALL microphonic? The tapping was done with a LONG, lightweight wooden skewer stick. This amp has a significant hum. It also has a crackling sound like sizzling that comes and goes, but is not obnoxious....yet. Any ideas? Anybody know a good amp tech in the Seattle area?