FIXED!
So after taking out the Alps pot, putting in the stock one, checking and-rechecking every single solder, I still had the hum. Aaaargh! I was about ready to toss the whole thing out the window.
And then I realised how silly I had been. When I had the panel painted, I sanded off a patch so there would be metal contact for the earth lug. But I neglected to do this for the pot shaft. I added a little wire from the shaft to ground, and the hum is all but gone. It's still there in a small amount at very high pot position, but this is well beyond normal / safe listening levels.
Happy ears now! Thanks for all the help guys.