I think if you shop around on the net, you can find the 45z for 300-350.00. I would not recommend it over the 40Z at 500, but at 325.00 I would.
I'm actually shocked that you were able to hear a difference between them, you gotta have some good ears!

Did you get a wiring harness? That is by far the easiest way to go. It almost sounds like you connected both to a switched source and not the battery. Hooking them both directly to the battery would result in you having to physically turning the unit off or it would stay on constantly.
Good luck!