Mount up floorstanding speakers, should we?

Hi,

I tried to mount my Tannoy (floorstanding) Cheviots last week on bricks yet they sound very different, in a bad way. The sound becomes lifeless, much of the bass is gone, sound stage is missing, imaging is poor, almost everything has gone worse.

I wonder if it's wrong to mount a floorstanding speaker. They sit too low (concentric drivers centre at least 20cm below my ear level!)even I am sitting on my sofa and that's the why I need to mount them up.

Thanks
Chris