Down-firing is a simple premise. You are using a reinforcing plane to boundary load the woofer. It's like the wall, floor, corner, etc. Identical to side-firing against the hull. Every time you fold another plane around a woofer you gain another +3 dB. It works. However, you need to be careful that the distance between the woofer and adjacent surface is not too small. If you reduce the radiating surface area at any point during the transmission path then you have choked the output and countered a portion of the down-firing benefit. You may even alter the woofer's damping properties. If too choked, you are simply adding more 'shaking of the boat' and transmitting through the boat's sole.
Venting an otherwise sealed compartment is essential for quality bass. Look at what you have with a sub/box within a seating or helm console. The expansive compartment and adjoining gunnel cavities represent a giant and compliant air mass. This air mass is like a bottle. The vent is like the mouth of the bottle. The smaller the vent the higher its resistance to flow. The larger the vent the less resistance to flow. With a tiny vent there is far less impetus for the bass energy to flow from the compartment and much of it is consumed within. It's not a linear function. The upper bass (attack, transients, midbass) is severely filtered. Also, when the sub/box is placed within another enclosure you have seriously altered the phase response so integrating with the coaxial midbass is not nearly as seemless as direct radiating. More bad news. If you don't vent the compartment then oftentimes the coaxials sharing that compartment are the medium where the bass pressure is released. Now the coaxial midbass driver is being modulated, acting as a sympathetic radiator, out of phase, and in a region it is not supposed to play. Now the midbass and midrange on that side of the boat sounds crappy too.
As a test, listen to a sub in the lowpass mode in total isolation and without the fullrange contribution. It will sound slow, soggy and drunk. This will demonstrate how dependent you are on a good interface between the sub and coaxials in order to achieve decent bass tonal construction.
Some are going to be completely satisfied with 'hit', 'hammer', 'thump', 'bump', 'rumble' and all those tactile and percussive descriptions of bass. The more the better. That's not music. That's turning a reproduction device into its own monotonous instrument. Do it right. Go for something that bumps AND maintains musicality.
