The chamber is there to allow you to "mass load" your speakers. By adding lead shot or dry sand, you add mass to the speaker, which should dampend cabinet vibrations. It also purports to reduce "doppler distortion."
Does it work? I dunno. It seems to me that it does- in my place there are two pairs of Monitor Audio Bronze 3's upstairs, one loaded with 25 lbs of #9 lead shot and the other empty. I think the loaded pair has better bass. Lest WmAX or Mtrycrafts jump my *** over it, no I haven't measured or DBT'd it.
I say it's worth a try. Lead shot is reasonably cheap (about $15 or so for a #25 bag, depending on the size of the shot & where you buy it). Sand is "dirt cheap," so to speak.
If you go with sand, best you pour it into metal or glass pans and dry it in the oven- if you have a lot of humidity in the sand it can rot your speaker cab out over time.
