Hi John,
I have the same receiver and don't need more power to drive my Atmos setup. (At least not until I try it as 7.2.4) Did you try the new speakers with the AVR-X4400H? I assume is isn't loud enough for you or you wouldn't be looking for more power now.
You would purchase a power amp that does not have a built in pre-amplifier stage. Such a device is called an integrated amplifier. You just need a straight old power amp.
I hope this is helpful.