Well, if the dust covers are not done right, they can affect the sound.  In the early days, I made my dust covers as tight as the diaphragm, and those indeed affected the sound, especially the bass.  On the newer headphones I make, I crumple the Mylar and only give it a light stretch to make the dust covers.  And they don't make any difference in sound.
 
	I happen to own a pair of Martin Logan.  I feel that the integration of their ESL and the subwoofer is not that great.  I very much perfer my DIY Full range ESL.