I guess I should be a bit more clear on what I am looking for.
I am familiar with isobaric subs, and the technical reasons behind them.  I understand why ACI designed the Saturns - drivers of that vintage required a huge volume to work properly.  Modern drivers, not so much.  Drivers have come a very long ways from the late 1980's.  What I am really trying to figure out is should I try to get 4 "new" drivers that will be compatible in my current enclosure, or should I find 2 drivers that will be equally as good?  I have not looked into components for ages, and am totally out of the loop on what would be good today.  I also understand that 4 drivers will require a lot more power than just 2, which is why I am running 3 Adcom amps currently, 2x GFA555 for the Saturns, and a GFS545 for the Sapphires.  Plenty of power there.  Plus, if I "have" to, I can go buy another Adcom quite easily - always many listed for sale.
So opinions?