subwoofer buzz - how to eliminate it

Convert to front firing. 

@grislybutter Turning sideways may or may not help. You'd have to try it. Raising the height using a platform helps whether it's front or downward firing. Personally, I wouldn't bother with the slab, even though it's probably a good solution. I would just put the Nobsound springs on top of an Auralex Subdude platform for excellent isolation and a little height, and also play with placement if you are so inclined.

Well, it could be that the sub is particularly badly placed for that particular frequency.  First just try moving it more into the room as an experiment, see if it not only stops shaking but also just sounds better. :)

It's in the corner so I will move it, turn it sideways, try the simple things first.

Moving the sub out of the corner may also improved the sound of bass in your room. In his great series of videos, Art Noxon of ASC talks about the high and low pressure areas in the room: standing waves build up along wall/wall, wall/floor, and wall/ceiling intersections, corners being the most problematic. ASC even makes a bass trap for use under subs, which sounds counter-intuitive but actually isn't.