Personally, I know just what you mean about the bothersome resonance.
A close friend of mine who is probably the most respected loudspeaker modder in the industry almost ALWAYS makes addressing this issue a prime focus of his efforts. He uses Dynamat (whatever the best one is these days) on the inside of the cabinets. However, he cautions to not use TOO much, as you don't want to remove the soul of the music. The Dynamat will give you superior dampening to the fiberglass rods.
Needless to say, he's a very creative and talented person. One who eschews the conventional wisdom in favor of what he has found to produce the best sonics.
