Agree with SpaNE...
Thats exactly what we did @ our last house. It worked great. Much better sound in my opinion. The only downfall I can see with this option is lack of ability to adjust volume once you are in the tub(not a problem for me tho).
Even if you don't want to run off your receiver, you could pickup a 2nd cheap receiver dedicated just to your tub, and still be well ahead of the price for a pre-instaled tub stereo.
I know this is the direction i am gonna go when i get a spa for our new place.