I have had four damping discs including the Marigo green and black and the Herbie's super black hole. The same results with the Herbie's, dulling the dynamics and highs, robbing the sound of it's musicality as Robert and Frank report. Yuk! However, I am using it on my VPI TNT VI placed on the center spindle where it raises the label slightly and with the VPI heavy weight, gives my less thick, standard discs (under 140mm thickness) a better VTA. I suspect that the center dip in the platter is compensated for by the Herbie's black hole.
I also have the Ultradesk CD cutter and a first edition of their demagnetizer. The only, major and simple to use product I've been using for about 8 years now is the Walker Talisman demagnetiser, a pair of magnets swept over the top and bottom of discs. It's so obvious in use, just play a disc, then demagnetize and voila, a superior sound (more coherent, dynamic, clean). Sure it may sound like a rip off at over $200 for a pair of magnets but it's elegant and better than the Ultradesk cutter in so many ways. I also highly recommend Herbie's tube dampers. Even multiple silicone rings on tubes are great at reducing ringing/vibrations from tubes.