As far as keeping the older sub in the system, I would as long as you aren't pushing things to their limits....like was said perhaps it wouldn't keep up. Should generally provide more even coverage in your room (placed appropriately) even if not a matching unit. At least that's my experience (unmeasured, tho) with adding 1 and 2 Infinity PSW310 subs I picked up on sale, in addition to my Empire. I have a big room, bigger than yours, just the basic room let alone open stairwell and adjoining rooms (altho the adjoining rooms have sliding doors, often at least one is open), so more pressurization is a good thing IMHO even if not matching/ideal. Right now I've just gone from the Empire and both Infinitys to just one and it's still very nice (but really haven't tried pushing limits a lot either, or measuring).