For instance, four with 400 W on tap in 20 ft^3 tuned to 16 Hz has enough theoretical peak SPL to either blend to some room gain and/or EQ it flat all the way down or meet THX cinema reference down to 25-30 Hz. Trade some peak SPL for more damping to further 'tighten' it up to better mate to horns.
Ah! So it is you! The City of Bergen didn't 'ring any bells' with me.
Merry Christmas to you also! ('Sounds' strange nowadays, what with all the political correctness 'forcing' folks to use 'Happy Holidays')
Well, you know me, 20 ft^3/dual 15" is just a good starting point for getting serious, but your horns already take up a sizable percentage of living room 'real estate' if it's the same as you sent pics of several years ago.
Some more Hornresp numbers:
A compound reflex port/front horn @ about 220 Liter net has a nice impulse response in Hornresp. A bit bigger than a standard BR, but may be worth the effort to match with the Iwata/LeCleach horns mentioned above.
A 600 Liter tapped horn would average about 109 dB in the pass-band, and provide 106 dB @ 20Hz; this is driven to Xmax=4.5mm with Eg=14V. Not very impressive (?).
I recommend you fire up Hornresp, and give it a try there are just too many different results that can be achieved in a simulation.