I own a pair of B&W 801 matrix SII and use a pair of Krell Bass Alignment Filters in balanced mode and wow--what a difference! The bass is so much punchier and overall sounds a lot more natural and blends well with the rest of the speakers. I bought mine off Audiogon and I am happy with them. The only concern I have is that B&W does not have any more woofers or repair parts for the 801 series II and possibly the III speakers, so if you drive them to hard and damage the woofer (because of age) chances are you will not get a new one, or a repair kit. There are ads on audiogon looking for woofer drivers. Therefore, consider the age of the speaker before spending money for a filter designed for that model only, as the Krell is only designed for the B&W Matrix 800 series of the late 80s and 90s.