Acquisition memory is right on the ADC module boards, so it's either additional or larger chips on there (under the individual heatsinks), though and whatever kind of firmware switches are needed. There may be a configuration chip on the acquisition board, but the options are on an EEPROM on the PCI interface card, if I remember right. It's been found and analyzed to some degree in other threads, since this is the same general architecture as the 7x00a series.
Looks nearly identical to the 7200s and 7300a I've seen, with the exception of the timing distribution board in back and the frontend boards. The lower spec scopes have a smaller heatsink, a cage (where the thermal pad is), and mate straight to the front panel, and the timing board on your DDA looks like it's thermally/RF isolated with the can, whereas the other scopes I've seen have a low loss PCB with individual cans over things. Well, while the ADC boards are similar, they may be larger for the XXL memory option, I don't think the ones I've run into have had 6 heatsunk memory chips on them.