200 liters each tuned to 25 Hz with two 30x6" ports. Use enough power, 2 Crown XS900's for example. He won't need a high pass filter with music but if he really wants to push them with movies, I would consider a 2nd order HP at 20 Hz.
Use RL-p18D2's (voice coils in series and bridged amplifiers).
Should be plenty loud (=over 125 dB).
