There is another post (http://adadepot.com/index.php?topic=224.0) re vintage split stacks (I've not used them) which discusses a "bright" switch on the vintage cabs no doubt to make them sound more modern.
My understanding is that the vintage cabs are loaded with speakers designed to mimic the older style speakers (eg Celestion greenbacks etc). MJMP did the bright mod on his split stacks, not sure if his are vintage cabs (probably not if they didn't have the switch already). All that said, you can put a variety of 12s in the box depending what sound you want. My Boogie Celetion 90w 8 ohm 12s are quite bright (like most things Boogie
) and work very well in my split stacks. For me, impedance was also an issue as I wanted the cabs to be 4 ohm to get the most out of my ADA B200s power amp. Normal ADA cabs were typically loaded with 2 x 16 ohm speakers thus 8 ohm cabs (speakers wired in parallel).