Those little plastic things don't alter the sound that much.
But K601 earpads with K601 foam pads will give a more fullbodied sound.
I think this combination is hard to beat. It took me while to found the same appealing sound (with the MB Quart QP90).
I've ordered some k601 pads and foams, I'm not quite sure as to exactly where the foams are going to be placed but I guess that might be more self explanatory when they arrive, my guess is between the black grill and pads somewhere (might be a tight fit) please tell me if I'm wrong.
I've been enjoying them so far, it's also got me interested in what the K501/500 sound like in comparison but as I'm aware they are all have a lot of similarities with everyone having their own personal favourite.