I removed the OS-ROM from the 800XL board and installed a socket. Tested that ROM in the working 600XL and it was faulty. Testing the working OS ROM from the 600XL in the 800XL board and there was still no change so obviously more than one fault to deal with.
I have a lot of new 41256 RAM that I bought when I upgraded my XE's to 320K in 2007. Will 41256 work in the XL board? IIRC pin 1 had to be soldered to ground to use them so maybe not.
You can replace the 4164 DRAMs on a 800XL mainboard with the 41256 without problems - at all 800XL mainboards pin 1 is connected via pull-up resistor to VCC. Of course only 800XL, when talking about a 800XE, just must connect all pin 1 to ground or +5 volts.