Mass Loading of Proac Response 3

Wondering if anyone has experience varying the amount of sand in the cabinet of Proac Response 3's. My friend has a pair that he purchased on the Audiogon and is trying to figure out how the amount of material in the cavity may effect the sound--particularly the bass response. Thanks in advance for your input.
Ask Lrsky of the Gon, he may tell you of his impressions of mass loading speakers..Tom
I have the ProAc Studio 200 Floor standing speakers, I do not use the sand, but I will quote what the owner manual says.

"As a general rule of thumb,a small amount of sand will give the bass generally more warmth, adding more sand to the cavity will give a dryer tighter quality, most people will find that a half to two thirds full is optimum in most models."

The one thing I do recommend is use the spikes.Quote,

"Most floor standing loudspeakers come with a plynth which is bolted to the bottom of the loudspeaker and spikes are added.The plynths will improve bass and imagery by increasing the stability of your loudspeakers."

Just a note, if your friend does try the sand, use dry silver sand only.

Hope this helps,Jim