You will find that there is no issue for that combination. I know that because not only in theory, also in reality I have used my AVR-X4400H that has no preamp mode, and also used an AVR-X1800H in both preamp mode and non preamp mode with my buckeyeamp Hypex and Purifi Eigentakt amp that has only 25 to 25.5 dB gain.I am going to order the NCx500/3ch and give it a try. L/R will be Pre/Amp mode XLR and C will tbe XLR-RCA...we will see how that works. This is making me crazy enough to be considering the Marantz 8805 with Buckey Amps....and be done.
Just keep in mind you will have to turn the the volume up a few clicks, but to level match the channels powered by the avr amps, you should rerun Audyssey, or simply compensate the gain differences by adding 3 to 3.5 dB to the LCR channels using the level trim settings manually. After that just sit back and enjoy the music with the peace of mind that you won't hear distortions as long as the upstream devices and contents are distortions free (audibly speaking).