abboudc said:
Do you have any data that shows this? Some of the most respected speaker brands suggest lead and or sand fill in their cabinets and/or stands. If they were going to do "BS-Marketing", they could probably find better ways to do it than add another compartment to their speakers and have their customers put lead or sand in the bottom.
Most of the sites i've read suggest that the sand/lead/other dampening material isn't meant to stiffen every internal wall of the speaker or reduce vibration amplitude, but to give the overall cabinet more mass and weight, making it less susceptible to vibration. It seems to me that the intended impact is to plant the speaker firmly on the floor and minimize the vibration between the bottom of the speaker and the floor. Unless i'm missing something...(which i wouldn't mind at all if someone pointed out...i'm here to learn

)
It's still BS. A lot of companies do this to get the end user involved. It's easy and cheap for them to add such a modification, because most of the effort and cost is on YOUR end. In addition, when a customer does a tweak that is supposed to improve the sound, they will expect a change, so even something that doesn't really affect the sound will yield a positive reaction. It's amazing what bias can do to perceptions.
Regarding the intended impact, the added weight will help the speaker couple to the floor more effectively (as will spikes), but this will increase the vibration transmitted to the floor from the speaker. Ideally, you want the exact opposite: isolation from the floor. Vibrations in the floor cause sound coloration that are a detriment to the sound.
If you think you want to do the Peel 'N Seal and mineral wool mod, be warned that you WILL lose some bass extension and some efficiency. The mod decreases the effective volume of the cabinet and also attenuates the sound somewhat. However, to me, the gains from the mods far outweigh the costs. I personally noticed much less coloration in the midrange, specifically vocals.
Regarding the crackling issue: it may be an issue with the potentiometer in the volume control. When they collect dust over the years that happens. My dad has an old Yamaha CR-1020 that has the exact issue. A thorough cleaning (at an electronics shop if you're not brave enough to open it up) should fix the issue.