Thanks to all, I did contact Jim Powers, and he suggested vacuuming again with particular attention to the horizontal bars, which I did. The sound is now much much better. I am not sure yet whether I want to take further action, although new stators sounds appealing.
I have 9 year old ML SL-3's which I love. But slowly they have become bass heavy and I am required to turn up the amplifier more to get adequate sound volumn. I recently turned the bass down on the base of the unit and that helped the balance but I am having to turn the amplifier up to levels...