Upgrade Avalanche 18" subs?

I built a set of Avalanche 18" sealed subs in ~9 cu ft cabinets a couple of years back. Here's the build thread.

http://www.avsforum.com/t/1221102/teds-twin-avalanche-18s

I had planned on upgrading someday to a set TC sounds LMS Ultras. I see a member has 4 Aurasound NS-18s for sale in the classifieds. I can save 50% off the price of LMS Ultras with these drivers, it has me thinking of picking up a set.

I could just replace (with some cabinet mods) the Avalanches with the NS-18s or add them to each cabinet to create 2 multi driver subs. I haven't had the time to run WinISD numbers so I'm not sure what I would gain either way. What I would be looking to accomplish is to boost my sub 20hz output. I have two CE4000 to drive whatever setup I end up with so power is not an issue.

Is this upgrade worth the trouble and if so what configuration do you think would work best?

Yes, there is no way to seal off the room.

I ran WinISD comparing the Avalance to the N-18s. I was suprised to find that the two were almost identical in output. I'm not sure I would gain anything buy this upgrade so I think I'm going to pass. I think the better upgrade would be to add two passive radiators to each box if I want to increase the output at the low end.

Any suggestions on which 18" passive I should look into?