I can't comment on the weight difference, but as an owner of the 661, I have nothing but good things to say about it. It looks good, sounds good, and has a ton of features for a sub-$400 receiver. About the only thing I wish it had, was the ability to set different crossover points for each channel.
One kinda lame thing though is that it does allow you to rename your source inputs, but the display only shows it for a couple of seconds and then switches back to showing what soundfield you're using. No big deal. I just would rather the display stay showing what source I was listening to.