To support larger SIMMs, you need a modified ROM (which is why I commented that it needs the ROM SIMM socket populated), at least for Mac OS. I'm really not sure about A/UX, I haven't tried it with the modified ROM. It is likely that it uses whatever the ROM sets up, I doubt it reconfigures the memory controller. Reconfiguring the memory controller while running from the memory you're reconfiguring gets a bit touchy.