Moby -

If your buying a new HU that has a sub out, I'd recommend buying a dedicated monoblock style amplifier for the new sub. This will give you dedicated control over a part of your stereo system that needs it - BASS. If you already had a 5 channel amplifier I might agree more on the lines of what Sandm said and tailor a sub to work with whatever power you had available. But in your case, you need an amplifier when you buy the sub. Note, he's not wrong in what he's telling you... It's just a different idea.

From a DIY point of view I'd look at where I'm going to put the sub before getting one. My suggestion would be depending on your boat, to put it up under the driver console, ported box, down firing. A lot of guys just build a box and face the woofer outward, but what they don't realize is by down firing the sub (i.e. porting at the floor) you get the sub to acoustically blend to the floor space / console area and PRODUCE WAY MORE BASS!

If you need help with a box design, contact me at [email protected]. We do this all the time for guys.

Hope that helps,

-Brian
Exile Audio