I have a sivlerface Fender Champ and there is a loud crackiling sound that comes from the speaker. I have replaced the tubes and have ruled out the speaker as being bad. At very low volumes (barely audible) the amp sounds OK, but hit the strings hard or turn it up and the crackling sound comes out making this amp unusable at this time. I would run this amp on 10 (volume) alot of times with no problems until now. Any ideas what I should look for?