With that Ultra system and a pair of nice subs, you should be looking for an AVR or preamp that has MultEQ XT32, and that has x.2, where the ".2" represents 2 subs.
For future upgrading, you can find and AVR with preamp outputs. the best preamp outputs are balanced XLR, however, RCA is just fine. As mentioned before, power amps outlive AVR's and preamps. Mostly that is because of changing technology. You could buy seven 1940 monoblock tube amps of about any brand and connect a 2014 preamp to them.
Subxero, you rock buddy! You are one of the most helpful people I have seen. Nice write up.