I think I'm gonna hop on this sonotube bandwagon with 2 500L enclosures, each housing an SS rl-p18. power will come from an xti-4000 or 2 xti-1000's (both are $800, and supply the same rated power, 2800-3200. pro's con's of doing 1 vs 2 amps?).
Quesiton is, what tuning? I am considering everything from 9-15hz . Anechoic, the 15 looks much better than the lower tunings, but after room gain, the 9hz looks great also. How low, frequency wise, is group delay important? The group delay goes very high near tuning (50ms), and with the 15hz tuning, it is closer to most "usable" material.
As far as large signal response goes, they are the same down to 30hz. At 20hz, the 15hz tuning has a 5 db advantage, and at 10hz, the 9hz tuning has a 14db advantage. (excursion limiting is why those numbers are different from the fr graphs below)
For the room gain here, I just put in reasonable numbers for F3', and Qb' in my program. (Frequency at which there is a 3db gain, and equivalent q (how lossy the environment is)) I used 30hz, and .5, which lead to a Fb (resonant frequency) of 19.3hz for the room.