I noticed that you have a moderate to large size room and you need, although your speakers are moderately sensitive, a decent amplifying power to get to acceptable Sound Pressure Levels in your listening space.
Either the Marantz SR7011 or the Denon AVR-4300H would be an excellent choice to drive your speakers. They both feature a clean 125w/ch. On the other hand, your RX6s won't handle powers stronger than 125 watts continuously, and that rules out external amplication. By the way, the Denon is available from Amazon for $799, a real bargain. I doubt that you will be able to get the Marantz for such a price. With either of these receivers, you should have enough power. Should the volume be insufficient to your liking, you will have to get speakers which can play louder and accept more power.
I did a quick calculation of the SPL with sitting at 10 feet from the main speakers. With 125 watts coming out from the left and right front speakers, you should get a sound pressure level of at least 105dB. That is quite loud. A question, is the Pioneer powerful enough, and at what distance do you listen from the speakers?
Comparing the Marantz or the Denon with the Pioneer, I strongly believe that the recommended AVRs would perform better than your LX55. Amps are of a better quality, and most audioholics will recommend a Marantz or a Denon AVR rather than a Pioneer product.
The choice between those two AVRs will depend upon the features you are interested in. Both of them have Atmos and 4K features and are among the most reliable products. They are manufactured by the same company, D & M Holdings, which was recently bought by Sound United which is owned by Polk Audio.