2 sb13 ultras would load your space fine with opposing corner setup. Front right rear left. Your 1st order length mode is right at 20hz, so getting that extra room gain right where they fall off at 25hz will be a nice coincidence. Since your rear right is open to the rest of the L shape you will have to cheat your rear left corner sub away from the corner if you want smoother response for all listeners. Maybe 4' or so I'd start with and test from there.

They are on sale at the place you are thinking. Probably swing a better deal with 2.