Well, my Contessa is finally working again.
Turns out that one of the 6v6 tubes had a shorted cathode. Because of the circuit design, a shorted power tube will cause the one side of the rectifier (6ca4) to short out due to the current draw. Was pretty sure there was a bad transformer and I thought I had tested ALL the tubes properly during my own diagnosis, but I didn't catch the bad 6v6.
Thankfully the problem didn't ruin anything else in the amp. But since I did a full cap job on it prior to taking it in to a pro, this amp now SHOULD be reliable for the long term, and it sounds great once again!