3-4 hours of float on the house battery is quite impressive! you must not play your music very loud
But seriously, the music system that comes on our boats is quite impressive and it's well amped up. I added 2 batteries and they get me through most days - not Memorial day when we floated for 8 hours in Devil's cove on Lake Travis. But, that's a completely different story that
@Sotally Tober can tell you about
I installed 1 Interestate 27DC from Costco (slightly larger than the once comes with the boat i.e. 24 DC) - a great deal <$85 and comes with 1 year full replacement warranty. I also added a Crank it UP cell (I paid ~ $300 for that one).
I wired the batteries myself which was pretty straight forward (in parallel - positive to positive, negative to negative) to the house battery (the one in the back)
The surf series boats have the solid ballast tanks in the back which means you will need to install the extra batteries somewhere in the front. I found the best / easiest place to run wires to is in the compartment in front of the captain (right under the WS amps). Running the cables wasn't difficult (especially if you buy a $15 fishing tool from Amazon). Run the cables from the Port side where the batteries are through the engine bay to the Starboard side (run along the other cables using zipties) . Then run it to the front through the hull. Remove the cupholder to the right of the captain chair and that will be a great midpoint. From there you're 5 ft to the destination which is accessible from the little door in the front
Having the batteries in that location makes adding another amp / speakers easy since you can just connect directly to them and get remote signal from the other amps already installed. I am planning to add 2 x Revo 10's before this weekend (people will listen to my music!! lol)
I hope this helps.