Re: Feedback problem
If the tubes of your amp are old (really old, approx +2 years)it may cause this feedback probs.
The cable prob is another, if they are too thin and would make noise if you shook them, change them against another one. Check the plugs also, I have had some which are a little loose and causes some problems with sound and feedback.
There are tons of reasons, anyway you better check the whole signal way from the guitar to the amp.
The easiest way to do so is, guitar to amp direct wired, first cable - test - second cable - test - pedal to amp - test with different gains and volumes w/o guitar - and fianlly guitar-cable-pedal-cable-amp. So you can find where the problem is.