Big,
It reads like you do not have tower speakers so I'll just assume that is the case. If that is the case then an EQ would have limited value as a zone control or RCA splitter.
You should be able to use the preout pass-throughs on the JBL amplifier to feed the RCA inputs of your new amplifier. This only requires a short RCA jumper from amplifier to amplifier. The advantages would be a) constant bass regardless of the fader position, and b) most amplifiers (not all) have an independent OP amp driving the preouts so there is no real division and no loss of preout voltage. This provides the advantage of having a bow to cockpit fader if occasionally you want to turn down the bow and use that as more of a quiet sanctuary for the little people.
Mike's (MLA) suggestion would also be great if you wanted to use the fader as an active bass level control. You can always do a fixed gain balance between the bow to cockpit speakers on the amplifier. Most amplifiers (not all) have a configuration switch that allows all four channels to be driven by just the two front RCA inputs. If your HU is on the port side then the value of this feature is a little diminshed because the control would not be readily accessable to the driver. But it sure would be handy if the HU is on the starboard side.

David
Earmark Marine