Your best bet for controlling the sub would be with a product like the SMS-1 or a Behringer Feedbck Destroyer. This is probably a hack-job of an explanation, but the basis of putting acoustic panels would be to deaden reflections and/or standing waves in the corner, so the products would go on the walls, not the sub itself.

The grandaddy of those products was the ASC tube trap (they're still around: http://www.acousticsciences.com/), but they kind of look like a giant roll of paper towels.

So what would the benefits be? In terms of the sub anyway, if your response was anywhere like mine, you probably have a big bass hump somewhere in the region of 30 - 60Hz, which means you're getting boominess in the sub's response which, in turn, influences how seamlessly it integrates with your mains.

With your mains, the panels from ATS placed behind the speakers would probably help any ringing problems from the marble. Problem is, it would f-up the look of that beautiful fireplace. But that may or may not be an issue for you. Do you have any hardness or is the room too "live" sounding? By that I don't mean like live music, but an echo chamber effect.

Lastly, of the two acoustic treatment products, I think the ASC Tube Traps are more ideally suited for standing wave problems and use in corners.