another option that I have seen before are those little motorcylcle can speakers.
I know you have all been at a light and heard a harley come up beside you with a soundsystem that sounds like it belongs at a concert hall...
Those speakers are small and pack a wallop. Cheap too.
That all said...I had crawltunes on my last buggy and they were loud. But the cans broke in a hard roll over and they ejected themselves from the buggy (but I do know that they have been changed since then...i am not knocking his product...they worked great and I was happy with them). Now I just use a bluetooth waterproof speaker box that sits in a little mount between the seats. I can take it with me to camp, in the shop, at the pool....cant beat it in my opinion. Not sure I will ever have another buggy specific system again