Apparently you are new to the scene because McIntosh is famous for reliability and customer service. I had a 6500 for a while and it was awesome. A buddy of mine had one too and it powered his Revels in big room with surprising ease. It has a lot of power despite the modest ratings. If you have speakers with proper resolution, it will be a huge improvement over your 840 (as it should be for the extra cost). I think the 6500 is one of the best deals out there.
Arthur
Arthur