First, 15 feet is far too much distance between those speakers. Try 8 to 10 feet and the stands will certainly extend and smooth the bass response. "Book shelf" speakers nearly never sound their best on book shelves. The bass of the speakers make the shelves, and everything else on the shelves resonate adding colorations, bass muddiness, and/or one-note bass qualities to the sound. Getting out of the corners also will improve the sound because the corners will act as a horn, adding a slightly shouty quality to the sound. To demonstrate the "corner effect" of the sound have a friend stand across the room and say something. Then, have them repeat the phrase with their hands on either side of their mouth (to act as a megaphone) ignoring the difference in volume, notice the changes in the character of the sound (tonality.)

I have a pair, too -- and cheaper stands will work just fine (though you definitely want metal, not super-cheap MDF.) Of course, the B&W stands look great, and if you have the $700... BTW, you should experiment on keeping in or taking out the port plug. Taking it out, as B&W recommends, gives a little mid-bass boost; I leave 'em in and use a subwoofer for 50 Hz down (which, come to think of it, is another use for your extra cash on hand.)