Hi KC Very few things are etched in stone as you know...However, this sounds like a classic tube problem:
Here is a little thing you can do: turn your gain/preamp volume way up, and slowly turn your master/poweramp volume down. If the noise stops when master is turned down, your in luck, preamp(s). If the noise persists, output tubes or worse. Hope that helps.
A ceramic coupling cap,may have blown, open her up, (Take care of course), look for any burnt (blackened) or dry joint ceramic caps, especially near the power amp tubes
Just In case you need it, here is where to go for a free Peavey Valve king manual download in PDF format.
http://www.peavey.com/assets/literature/manuals/
00575730_10.pdf
Fenderphil