The Kicker 12L72 is a dual 2 ohm voice coil sub, so wired in series, it will be a 4 ohm load on the amp. Did you just buy all this gear? If so, I would exchange the L7 for the L3 as it will be a much better match for the HT-6's available 300W rms. Or, exchange the Ht-6 for an HT-4 and HT-1. This option would be the same power to your Pro-80. Although I would like to see 750 to 1000 watts rms to an L7 12", 600W is far better then 300.

You can scratch off the 12w6 from your list. They come in 4 ohm dvc only. You would have to wire it in series in order to bridge it on the HT-6 and it would only receive 220W. That sub deserves 500W or more.

4ga cabling would be the correct size for the HT-6, but the issue with an automotive amp install kit is the 2ft of ground cable they provide. This wont cut it in a boat because both the POS and GND need to terminate at the battery/switch. In a car, the whole chassis is a ground plane, hence the short ground cable. You are better off buying the cable in bulk. Also, buy the RCA cable based on the length needed.