Increasing the amount of absorptive material in your room is, in my opinion, more likely than not a step in the wrong direction. Nearly all absorptive materials are more effective at short wavelengths (high frequencies) than at long ones, and it sounds to me like the last thing you need is decreased high frequency energy.
Check to make sure the tweeters really are working. It is possible that something got jarred loose in shipment, yet the midrange drivers still put out enough highs so that you can hear the cymbals from up close.
Assuming the tweeters are working fine, unless your room is way overdamped already and your tweeters beam severely and are not aimed at the listening position, I'm stumped.
Back to the issue of room treatment - personally I prefer diffusion to absorption, as diffusion preserves high frequency reverberant energy while avoiding a strong, distinct early reflection (which is detrimental to imaging). Specifically, I use fake ficus trees in the first reflection zones.
Duke