Comparing a direct-radiating woofer under the helm to a woofer closed in an insulated locker, there can be a significant difference in sound quality and perceived output. A closed woofer is like cutting your amp wattage in half. They tend to be less accurate sounding and radiate through the boat more. More of a boat shaker, then a bass maker. A direct radiating woofer, you hear more of the bass, compared to feeling it.

Under the helm would be the spot I would want to go first. The results are typically better. if I was going to place a woofer under the helm, it would be a 2nd woofer to compliment the primary woofer under the helm. if I was only going to do one woofer and it had to be in the closed locker, I would suggest venting the locker in order to gain some of that radiation back.