blues junior problem

[matc]

I'm not sure if there's a problem with my amp but when I cranked it a little bit the bass gets really ugly. The sound has no definition, it sounds like a fart. It's more noticeable with my fuzz on. The amp is plugged in an avatar cab loaded with vintage 30's ( I removed the original speaker from the amp). I did the Bill m mods on it by the way. Is there something I could do to solve that problem besides buying a new amp ? I've seen a few videos and people don't seem to have this problem. Thanks !