I can't remember which amp is was now, but I do remember doing the same thing. I was just patting myself on the back after doing a really tricky solder job on one of the power filter caps.
I was feeling pretty good, like a surgeon that had just done an amazing bit of life-saving work.
Then came that "Lizzard Poop!" moment when I realized that I had it backwards. You can check, check, and then check again but sometimes when you are trying to get that thing in a tricky spot, it just gets turned around somehow.
Good thing yours was in the output stage. If you had fired it up with one backwards in the Power supply you might have seen some nice fireworks.
I know what you mean about building it yourself. I've owned more amps than I can remember but I've never enjoyed any amp more then the ones I've built.
Revel in your build!
