If you have the room, a third battery for stereo works best. If you have a amplified system of more than 800W then your definitely going to need a third battery...I recommend you go with something in the 92AH-105AH deep cycle battery range in the 27 to 31 group but really depends on your stereo's power requirements. Not sure how long you run the stereo but definitely get a deep cycle as it allows for a lower discharge vs recover rates...an AGM style would be the best but they are pricey. Not sure if you can get them there but look at the DEKA battery's...they have some of the highest AH series for the money. I'm running 2500W in my 268SC with them and can run for several hours and a pretty heavy load. Found the DEKA's to be far superior to OPTIMA's. Their a piece of junk IMO...I killed two of them in one season but my DEKA's are going strong now on 3 years and their cheaper. As for the socket 25 amps is pretty high...you could fuse it for 30 amps but your wiring would need to be upgraded and you may melt the sockets terminal end. Most sockets are in the 5-20 amp ranges. I would probably use something I could connect straight to a battery.
