Like said previously:
– the floor could be suspended wood, so susceptible to resonance
– the roof is sloping, so what about reflections…
I have the speakers under one roof slope.
@Paulssurround said that speakers in the other way could be easier to manage, with a wall behind I suppose (a nice listening experience with one of his friends system).
I can talk only about my experience…
1) The floor:
If your house is recent, it could also be wood or carpet on concrete.
On an old light suspended wood, the best I get for the moment is to use one 60x60cm Didond* 3 mm plate under each speaker.
*polyethylene surrounded by two aluminum plates (rigid support used for photo print)
I have listened with and without, and it's a lot better with:
– it allows the weight to be well distributed on 7 or 8 floorboards,
– it isolate the most part of the vibrations
2) The roof slope:
If your speakers have deep bass, I had to put them more inside the room (a lot of space behind) to calm down. I did 10 cm more last month for the best.
3) the roof height:
4.5 meters at the top at home.
In the house before, it was not under the roof and with 2.5 meters.
The space above 2.5 meters seems to help with reflections.
4) the windows:
At home, not a lot of windows near the speakers, that should help.
Thick curtains in front of the vertical window.