I like older, beat up cabins. Nothing like a pair of speakers to spruce them up a bit.
The M5s are wonderful! You will hear the spaciousness of your M100s and perhaps even better imaging and soundstage. Your room is very similar to my living room where I get a ton of enjoyment out of them.
I drive them with a Pioneer A9 which has about 10W more continuous out of it than the Marantz. But...and this is a big but...I can get a good 200W peak out of it per channel because it's got a toroid for each channel with almost 40,000uF of capacitance. This is real important because the M5s are about twice as hungry as the M22s. That HP driver will give you awesome bass but due to its mass, it's got to eat.
I sit 10 feet away and that Pioneer is just fine and gives me music throughout my 1075 sq. ft. main floor. Given your 7 foot distance, I think you'll be just fine with the Marantz.
Just check to make sure it has a sub out or at least pre-amp outs because you may want to add a sub later. Not because the M5s don't have enough bass but you may find they have too much and you want to manage it better through a sub.
