bass bottoms out, boat system

I have a kicker 12" comp sub (older version) specs say  4 or 8 ohm, (watts peak / rms) = 300 / 150, equivalent volume = 4.50 ft3, net displacement = 104.9 in3.

I have that hooked to a power acoustik ps2-520 (2ohm stable)specs are:

2 channel, 4ohm=125x2, 2ohm=150x21ohm=300 max 520. This is in my boat. Its in a box that has about 1.8 cu. ft. of air space. Problem is when i turn it up and hit some low bass it bottoms out. Just wondering what the problem is with that. Other than that it sounds good.

Thanks.!

i believe it is a single voice coil, not absolutely sure though. My gain is about 3/4 up. I also have a subwoofer controller on the head unit. If i am reading the spec off kicker correctly it says volumes is 4.5 ft3. The box i have now has maybe 1-1.5 ft3. Im not sure how old it is, its round, with the K on it and it has a big 25 on top for 25th anniversary. Hope this helps.

Thanks!