The Arcam AVR850 is over twice the price of the Pioneer SC-LX901 and has less amplifier channels, (Although in your case the extra channels appear to be overkill) so you have to be an enthusiast to go for it. I would look at the AVR550 which has all the features of the AVR850 but has A/B amplification instead of Class G, and a little less power, but is a similar price to the Pioneer.
The beauty of the Arcam is that it has Dirac Live room/speaker optimisation, which puts the competition to shame, (Exception are the Anthem Receivers which also have top notch room/speaker optimisation (Although a little behind Dirac Live) in fact, it is that good, you could have 3 different manufactures speakers across the front, and once optimised with Dirac Live, they would sound seamless, so you don’t need top notch speakers to get a great sound, but if you do, then it takes you to a whole new level. (Dirac Live is true pro system (And is used in professional installations) not a consumer based system like most of the others)
Hope this helps
Bill