Hey all. My SVT3 Pro has an issue. When using just the power amp side of it all is good, but when the preamp is used the sound comes in and out and crackles a lot. You can give the amp a little whap on the top and the sound comes back in. I was going to try replacing the tubes first and hope it goes away. Anyone else experience this? The tubes have been in there a while and it gets quite a bit of use.