Bass and treble what? Amplifiers and active filters for woofer and tweeter? Bass and treble tone controls? Single channel mono or stereo?
You must decide how much power you want and the size of the battery. Since a small battery will produce fairly high power for only a very short time then you need to determine the playing duration needed for the battery. Disposable or rechargeable battery?
TWO batteries for +/- supplies. And that is not counting the backup generator and propane tank. This solution fits perfectly within the information given in post #1.