I own a Carvin MTS 3200 Tube amp and I'm having some trouble with it. I bought the amp broken so I knew I'd have some trouble with it. I fixed the initial problem which was the input, but now the volume swells like crazy. I thought it was the power tubes so I replaced all 4 of them to no avail. Is it the 12AX7 pre-amp tubes, or is it a problem inside the amp?