It should help to reduce cabinet resonance. If you do go down that route. Make sure the sand is dry and you put it in sealed plastic bags. I'm not familiar with the speakers so I have no idea what they'll sound like.
I sand filled some old Missions and it certainly improved the bass. A very cheap and worthwhile upgrade I would say. I oven baked some kids play sand until it was free flowing and filled the plastic bags supplied with the speakers.
On the downside it makes them bloody heavy and a bit difficult to shift on ebay as I couldn't post them!