I have an older AVR 2311 and bought an Arcam SA30 for stereo .... it certainly has more "control" ... mainly the bass.
It almost felt like the AVR was designed more for LFE effects and with some songs like Freya Ridings - Lost Without You - where there's a lot of low notes it was very "rumbly" .... more like the sort of effect you'd expect for an explosion / thunder in an action movie and made me wonder if that's what the amp was more designed towards.
The difference isn't chalk and cheese and it's only with some tracks but it's certainly noticeable, I did think about selling the Arcam after a while but did a back to back using an amp switch and decided I'd definitely keep the Arcam as I preferred it for stereo overall.
I know it's not a direct comparison as I haven't tried the Marantz but might give you some idea the difference a seperate amp can make.
If it is a direction you're thinking of heading down and want to keep the AVR for movies, you may want to look at different amps as I don't think PM6007 has Home Theatre Bypass so you can use the same pair of mains for movies and for music.
I opted the other way when I had passive speakers - ended up with an Onkyo TX-NR818 amp, which I chose over several integrateds of the day - Leema Pulse, being one and a couple of others (I think one was the Exposure 3010S). Ended up being a great choice. Ample power for my needs - I think it had 120wpc in stereo only mode and worked a treat for my then Tannoy DC4s. The additional functionality was a bonus and it worked very well with the Cambridge 752BD universal player. This was back in 2013 and while the other amps were good, they didn't really offer a major step up.
With your choices, I'd have a listen if you can. Or at least compare the specs - power, damping factor, etc. It's not just about the watts, but how the amp controls the speaker driver movement, etc.