Sometimes the reverb works great (albeit very "surfy"). Sometimes, out of nowhere, it screeches at an unbearable volume. If I turn the "Reverb" to "0" it stops. This is intermittent. Today I turned my amp on, it was humming loudly. Even with no inputs plugged in or either volumes turned past "0". I finally turned down the "Reverb" knob and the humming decreased, and then went quiet, when I got to "0" on the "Reverb" knob. Any hints/ideas/clues as to what is going on here?