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
"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