Bill, on both our Rubin and my Kristall (when I had it) we cut out some panels of thin (but non-hairy and bits-free) felt and taped them inside the bass board using masking tape. Two similar sized panels can be made to fit quite nicely. Masking tape lets you try different materials, it being readily removed without any residue.
We experimented with various materials both porous and non porous e.g. rigid plastic, flexible plastic foam (NOT styrofoam obviously which falls into hundreds of alarming bits) cloth etc, but found that fairly dense but thin felt was easily the most effective and in the end we covered all of the holes. Black felt was best as it did not show through.
Like you we were quite bothered about whether any harm to the instrument might result (should have asked on here eh) so we were happiest with some porosity in the felt for that reason too.
It was pretty successful in damping down the bass sound although I suspect the effect unsurprisingly was more pronounced on the upper frequencies, than the lower ones, the opposite to what we wanted really.
I removed the panels from the Kristall before I sold it. With the Rubin, which is not often played, we removed the panels too in the end. Somehow, after a few hours playing, the heavy bass seems to moderate, don't know why. Maybe you simply get accustomed to it.
Anyway, good luck and happy playing with your new one. They are amazing value and really good little boxes we think.
Regards, Rob.
