There's no replacement for displacement.
That being said, subs in too small of a box won't move properly, and a port, in practice counts as more radiating area. From my testing, the limiting factor for me has been box volume. For example, the difference between 2 18's in 8 cube vs. 1 18, or 2 12's in 8 cube is only around or less than 1dB. The old Fusion Jonah Lamu (sp?) woofers were quite nice, but as far as I know most of the rest of their lineup was pretty weak. I had a couple of their big surfboard amps that made good power but liked to randomly self destruct. Most of their lineup is very similar to audiobling. All show and no go.
Figure out how much space you have to work with and how much power you can supply and plan around that.