Speaker Problem?

Was kind of hoping the resident sound system junkies can help me out here.

I've got a 2008 X-2 with four JL tower speakers on it. When we're not riding, we hang out alot at the local sandbar. Recently this year, I've noticed that I can't turn the volume up much passed about level 16 when the boat is off, otherwise I loose my the sound quality on my speakers. Normally when we're riding, we play music at around level 25. It used to be when I shut the boat off and left the music going, the clarity would stay the same, and we could blast it like normal, however lately that is not the case. When the music goes passed volume level 16, its like the speakers start to sporadically gurggle and lose audio quality, or the sound slightly pops in and out. However, when I start the boat, I can crank it with perfect quality. No speakers are blown, as the quality when the boat is on is fine, however when its off, it seems like the amps aren't getting enough power or something lately? I assume its a battery issue? Maybe time to replace my batteries, or at least give them a real good long charge? Or is there some other issue that I'm oblivious to at the moment?