During that Supra/Moomba era it was common to see two 4-ohm speakers wired in parallel to each of two channels of the Kenwood HU. If that is the case on the four cockpit speakers, then this needs to be converted from parallel (2-ohm load) to series (8-ohm load). In theory you would get more power at 2-ohms. But at 2-ohms the BTL type HU amplifier is unstable, will fall flat on its face prematurely and particularly hard. In actuality you will play much louder at 8-ohms versus 2-ohms with the HU. The bow is typically a dedicated speaker per channel for a safe 4-ohm load and is not the problem.
Sounds like your issue is compounded by low voltage.
In any case, eventually you really need an external amplifier for the six in-boat speakers. As it stands now, In order to keep the HU from hard clipping first you have to inordinately gain up the tower amplifier which raises the noise floor. Plus, without an external amplifier you do not have full zone control. You can run six speakers with a two channel amplifier in a series/parallel configuration and have a fairly even distribution of power. Although, you are better off with a four channel amplifier for the in-boats.