With each and every reinforcing plane you pick up an extra 3 dB of output, at least initially until the bass radiation escapes the impact of those immediate surrounding planes. So a side-firing woofer/port will gain a little acoustic leverage in the lower bass. However, there is a little storage and filtering of the midbass transients as a result of loading into the under helm cavity. With direct radiating you will pick up some articuation (depending on how low/high your crossover point is) and sacrifice a bit of the low bass reinforcement. There is always going to be a slight trade off. Generally with a 15-inch you can always use a bit more transients so the direct radiating method might sound better to you in some respects. Also, it's hard to get a sidefiring woofer/port when using something as large as a15-inch, stood off the hull surface by enough distance so that the bass-reflex tuning isn't affected. So that might be another area, depending on how tight things are presently, where you might see an improvement.