Bill, Thanks for replying. I want to do as little 'damage' as possible to these awesome horns. Based on my sand-bag experiment, I think they can be effectively damped and provide superb sound quality. I too would prefer a solution with minimal aesthetic damage. The box that you built around the back of the horn - was it built up to the flange or did it go up to/over the lips. If not the latter, do you hear any remnant ringing possibly coming from them? Did you cut the fins?Also, are you using your 511B's with a VOTT style cabinet or other type. What type of crossover are you using? Any HF compensation?
Regards,
AstroSonic