Hello,
In a room that size I'd expect stand mounts to almost always sound better - *if* you need more bass, a good sub kept on a tight leash will likely offer tighter and more room-tailored bass than floorstanders, especially the 684s.
I've had the PM7001, and the PM7001KI, with the matching SACD player too, for a time. I would strongly recommend that you avoid Quad and Wharfdale speakers with the 7001 series amps - these particular Marantz components need something much more excitable (like the 685s) to liven up the overall presentation. Having said that, I really don't like the 685s with anything I'm afraid, so personally I would suggest something from Epos, Dali, Monitor Audio, Usher or even Klipsch to help add some sparkle and kick to the super-smooth 7001 sound.
On a more positive note, while I am saying that these Marantz units are very smooth, they are also very "high-quality", and I expect you could spend up to £1000 on speakers and not humiliate this amp, if the sonic characteristics are complimentary. ATC or a highish-end speaker from Focal might be good, if you can outlay that like of cash.
Hope that helps. Of course, a home trial is what you need, ideally!