Your sub has to have an enclosure to work right.

You didn’t list the resistance of your speakers or the output of your amp at various loads.

Ive never heard of the amp brand.

Here’s my guess, your amp isn’t very good, you may be maxing it out turning it up and getting distortion, your sub also isn’t working properly with out an enclosure.

Budget amp but should be workable. As mentioned already, you really need the enclosure for your subs. 8’s don’t need much. Free air is going to be awful.

You mentioned that each song needs its own eq adjustment. Is the HU set up correctly? I used a Kenwood marine HU and it took a lot to get it dialed in. A better HU might be helpful.

Boats can be a real pain to get good sound. They’re not like a car where you have the shell to act as an enhancer to the sound. There’s a lot of loss.

The 8” speakers are component, not sub. The sub is a 10” in a box and it still had no ass. I spent 25 minutes on the phone with crutchfield today and just ordered a new Memphis amp. After doing some more research on the Rockville amps I’ve discovered that they over rate their power handling ability and just don’t cut it at the end of the day.

never skimp on amps. Good brands, on sale or clearance if you need to save a buck. The last time I broke this rule, it blew quickly.

4 gauge is good sized wire. However, 20' is a pretty long run. It's actually like 40' because 20' there and 20' back to the battery.

How many amperes does the amplifier draw? (what size fuse) You may want to up your wire size based on voltage drop

14 gauge speaker wire should be fine.

Honestly, stop buying the cheaper stuff. For a little bit more, you can get quality. It's cheaper in the long run, it saves you time, and the end results will make you happy.

Problem Solved