Gentlemen,
Last evening I finally had the opportunity to try a new tactic. I reinstalled the PolyPlanar powered subwoofer in the boat, except I added a 70hz 12db per octave crossover (to override the 200hz crossover built into the bass amplifier). WOW! What a difference! With only the 200 hz crossover, I had to turn the bass speaker down so low (to avoid upper bass boominess) that it didn't help the 25hz range at all. With the 70hz crossover, I was able to turn the bass amp volumn up to about 90%, enough to really help the very low pitches. Today I plan to try a 50hz crossover and possibly turn the bass volumn up to 100%. I'll let you know!
Regards,
Troy Scott