Speaker placement: Rug in front of speakers vs Rug underneath speakers

Well, OP specifically asked about the rug, so. Also, it’s lower frequencies that have longer wavelengths, thus being more affected by floor bounce.

See the data here: https://www.prosoundweb.com/images/uploads/TTPSWWavelengthTable.jpg

I would think more control would be had with multi surface treatments rather dependent on covering an area with one large surface. A large rug will absorb only high frequencies. In order to increase the bandwidth I would highly suggest a felt rug pad. They work well, ime. 

For floor bounce, draw an imaginary straight line from the front of the speaker (the center of the midbass driver), to the floor, ending at your listening seat at the same angle. Put the rug over the floor at that mid-point where the line hits the floor.

I have Sonus Faber Amati. I think in front is enough. But mine do not have a speaker pointed down. My intuition says up to the speakers is enough.

I use isoacoustic footers on my speakers on  hardwood floor with rug in front of speakers

Awesome speakers - I bet they sound incredible!  The SF trade marked "Zero Vibration Transmission" elastomer/spring decoupling system has been designed specifically for the 300+ pound speakers.  I would think having a rug in front should be sufficient sonically and that the decoupling system would be best supported by a hard floor, specifically for stability. However, at this level of performance it seems SF or your dealer would be in the best position to address the optimal speaker/floor interface.