In case it can help you, I come from a Denon 4306 bought in 2006, which amplified 7 channels, and with which I never had any problem. I currently have the same 7.1.4 speaker setup you intend.
Came to Marantz after faulty Denon x6700h capacitors, which was my first purchase.
Except for the sw, all the speakers are now powered by the Marantz, and they are slightly more demanding speakers, in a room close to 28m² and seated about 3.5m from the listening point.
The Marantz top has some differences with the Denon 6700, apart from the controversial hdam, its construction, the toroidal amplifier, or the 7.1 analog inputs, which the 8500 also has, but the rest of the Denons lack.
With the new 7.1 4 setup with height speakers, which I highly recommend, I have not had any power issues with the avr and all its amplification busy, and it is usually not possible to bring the volume above 60-70 / 90, it is not tolerable. I don't usually reach those levels, I don't like music that is too loud and usually I don't go above 40-50 depending on the source, which is already a considerable volume.
Without external amplification, both the Denon 6700 and 8500 - (or the Marantz 8015?) - are sure to be a good choice, more than enough for this type of speaker in a medium room. If you don't want external amplifiers at the moment.
Provide adequate ventilation for your receiver, I use fans to protect and extend the life of the electronics, although I have not really noticed problems of excessive heating with normal use.
Later you can try to amplify the front with some good boxes and have your own conclusions, I will probably do the same when I recover even if I do not see it necessary, difficult times ...
In the cloud a good class D without problems of noise, heat or space, a Purifi 1ET400A with 2 or 3 channels, very well valued here.