If these are anything like the speakers I have, there is no way they'll 
reach 500 feet.  They also use wide band fm, at least mine do, so it will 
be hard to find a different more sensitive receiver.  I'd get a wireless 
fm transmitter, such as the one from crane.  I think you can crank up the 
power on these and also could use a better antenna.  You could then use 
good sensitive standard fm radios.  I really believe that is your cheapest 
and best solution.

Please don't bother me with posts about changing the antenna or cranking 
up the power not being legal.  I already know that and really don't care. 
Just my suggestion.
73s
Butch Bussen
wa0vjr