Questions for my Boat Sub and Enclosure

[redthumper9]

First time poster here.......

Brief Background: I worked as an installer in the car audio field about 20 years or so ago. I grew up through the Punch 45's, Punch 200's, and Power 1000"s.....needless to say, a lot has changed since then. Last winter, I started my audio project in our family's Yamaha AR240 jetboat. The system consist of the following:

3 batteries

onboard charger

(2) Rockford marine 400d 4 channel amps (soon to be 3)

(1) Rockford marine 750d mono sub amp

(4) JL MX650 coaxials

(2) Exile audio XM9 HLCD waketower speakers (soon to be 4)

(2) 10" Rockford marine 10" subs (sealed)

I have bass....plenty of it, but very little on the bottom end so I am going to explore a port tuned enclosure. As a quick experiment, today I removed one of the subs and made a panel with a 4" round by 8" long port and screwed it over the missing sub location. I was surprised when my output was virtually the same with a noticeable difference in the bottom end. Now before I go any further, let me just say this is a BOAT.....I know it will never have the same acoustic properties as a car and therefore I will probably never get that ultra low end like I would love. But, I know there is room for improvement. The real problem lies with space. This is a family boat and space is a premium. The space that I will be using will allow me around 2.00 cu ft total (that is factoring in an average woofer displacement) but the enclosure will not be square or rectangle.....it will have an "L" shape. I have been looking a the JL 12W6, RF Punch P3, Sundown SA-12 and the Sundown X10. It looks like all of the ports these days are slotted. Is it okay to use round ports? the shape of my box is not going to lend itself to slot style porting. If I can use round ports, is there a calculator that you guys use to figure the diameter and length? My amp is capable of around 500rms @ 2ohm and 750rms @ 1ohm. Would a Sundown X10 compete with a SA-12? Any suggestions, please feel free to share