Hi Matt
I've got a broadly similar setup to BigBoss but with Pioneer VSX2021 (similar to LX55 without wireless networking) powering Monitor Audio Radius HD speakers. I understand that the tweeters are common to the RX series. I've not heard the Yamaha powering the same speakers, but IMO the Pioneer partners extremely well with the MA Radius balance.
I find that the Radius speakers are just that little bit polite and smooth compared to my Ruarks which major more on detail and ultra-snappy timing, and the Pioneer amp seems to bring out the best of the MA Radius, taking them by the scruff of the neck and giving them the oomph and attack they need. It's a beautifully fluid sound that makes you sit up and notice how good it is without being at all fatiguing. It always makes me want to turn up the volume to enjoy it rather than turning it down because it's attacking my ears, which some screechy hard-edged systems make me want to do.
Do try to demo though (I know this is stating the obvious) and just be careful about listening to a demo in a very different room size to your own. Speakers that sound great in an aircraft hanger-sized demonstration room can sound very different in a typical British lounge.
Happy hunting though!