Congrats on your amp. There used to be a guy on the web that had the knobs that you need. He used to sell on eBay sometimes too. Maybe he's run out of them by now.
The photo of the rear of your amp seems to show a big blue cap mounted on the outside of the chassis. Is this one of the new 600v caps you are talking about? If it is, it doesn't seem to be the safest place to mount it.