I want/need to add another sub and have 2 sitting in the garage to choose from. The boat (Formula 400ss) has a cockpit that is 15ft+ dash to stern with 2 subs near the helm. I have 2 locations near the stern I'm considering since there seems a very large drop off in base back there......which would you choose as I'm not sure which provides better "punch"? The new sub will be the 3rd sub powered off the 500w channel of a JL 900/5 (got the idea from a buddy who is already been doing the same for years). Both are infinite baffle and will have plenty of open air space behind them.

M3-10IB-S-Gw-4 10"....... This is a 10in sub rated at 175 watts RMS

The other is:

M8IB5"....This is a 8in sub rated at 200 watts RMS

Seems the 10in would move more air but is that true considering it's the "weaker" of the 2?

Are you planning on running all 3 subs off the same 500 watt channel ??? If so, you are going to have to be careful how you wire them for the proper ohm level or you will burn up your amp.

I'm going to copy what a buddy did 5 years ago on his......if his has lasted this long it must be right! LOL

Except different subs have different ohm ratings. You just have to make sure to stay above a 2 ohm load. Series Parallel Speaker Impedance
The 10" sub will most likely give a deeper base than the 8".

All 3 subs are 4ohm JL audio. (2) 10's and I installed the 8in this afternoon.
https://jlaudio.zendesk.com/hc/en-us...iring-Tutorial

You have to consider phase as well... uf you get them out of phase they start canceling each other out to some extent.