Re: What would you do?
Having been in your situation many times, I conclude the lesser of the 2 evils is building an entire new line of subs rather than toting around a ton of single boxes, though I'd hang on to some for any situations that a single or pair would suffice.
You might be interested in a change because you could get around 6 dB more output using the same power from a slightly larger single 18" enclosure.
Keystone Sub Using 18,15,&12 Inch Speakers - diyAudio
A pair takes about 3 sheets of plywood, they are as easy to build as your single 18".
No port compression, and the magnet facing out to the exit facilitates cooling, reducing power compression.
Some changes could be made to dimensions and layout without affecting performance.
I'd be surprised if you did not find that one Keystone equals the output of a pair of your single 18" when driven hard, only caveat would be using a cone stiff enough to take the load, which from your comments about the 18" you use should not be a problem ;^).
Art
