There is a lot of discussion on various boards about removal of loading caps to improve sound.
I've not seen any objective evidence that supports these claims (would love to, if someone has!).
Speaking with Bill at GPA a couple of weeks ago, he told me that he in fact has measured drivers before and after removal of loading cap, and said there is no measurable differences (i.e., it's not worth the money to buy the rear covers, screws, felt, etc.)

I have also read somewhere (caveat: it was Altec marketing literature) that said the loading cap helped to smooth out FR...I wish I had bookmarked that so I could remember where that was.