Ha, it's always the most obvious thing that ends up being the last thing to be checked. Probably should have thought to make sure all chips were seated earlier, but here we are.
Glad it's working.
(Side note: I can't for the life of me find the chip you mentioned on the 590 - where on the board is it?)
The continued autoboot failure is almost certainly due to the 4.6 ROMs. Older versions aren't compatible with anything newer than OS1.3.
I see 2 listings for v7.0 ROMs on eBay at the moment. Not sure where you're located, but hopefully there are some available in your area.