I recently bought a soundboks 2 ...battery powered box ....my intentions were to use it on my boat and not drain the battery .......it's not nearly as loud as I thought .......is there any way to add a small amp and a few 6.5 inch marine speakers ?....any advice would help thanks
That's a self contained system so adding anything to it isn't simple and will void the warranty.
Adding 2 or 4 6.5" marine speakers won't add much to the overall volume. It will spread the sound out a bit.
You would need to open the cabinet and determine what the power supply voltage is to the internal amp. That will be the voltage that the additional amp would need to be powered by. You could install an amp board inside the cabinet and speaker terminals to connect the additional speakers to the outside..
Since there are now more amps the run time of the speaker will be reduced. How much could only be determined by trial and error.