I fixed it.
I flipped the amp upside down to try to probe for the problem and it started working normally, so I knew it had to be a short somewhere. Followed the inputs but couldn't find anything. While scratching my head trying to find where the problem could be I absent mindedly flipped the fat switch and the noise instantly came back. I followed the fat switch wire back to the tube socket and found that the adjacent connection, at the second pin of the second preamp tube, had popped loose. Got it resoldered and now the amp sounds perfect. Even got rid of the mild noise when the volume is turned up.