Large panels do not lend themselves to horn loading. The idea of horns is to load the driver into a fairly small end of a tube, and make the sound come out of a large flared end. If you start with a very large inlet with low pressure, the horn will not work as normally intended. Throat area is too large. It will probably do something, but it may be more colorations than improvement. In addition, you will probably lose the open spaciousness that is a Quad hallmark, and will probably lose some other things too. For a small, possibly negligable SPL increase, you lose alot of what the speaker is good at. I doubt it will be a good trade-off.