Thinking of adding a second sub....? Advice needed

Current setup
2 pair of Rev10s / SD2
4 pair of xs650 / SYN4
1 xxx12v1 / SYN1 (in Wet Sounds angle ported box)
2 grp29 batteriers on stereo bank, and 1 starting battery, all with onboard chrager hooked up to them.

In the boat there is enough bass, but at the beach/cove its lacking bass behind the boat.

I may have found another SYN1, but have had no luck finding and old XXX12v1.
Anybody have the old XXX12v1 for sale? LOL

So my thoughts are get the used SYN1, and buy a new XXX12v2, and build the dual slotted Wet sounds box and put it under the observer seat. I always open that up at the beach to keep the amps cool.

Thought guys??