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