High pass filter with Sub

Does anyone have feedback on adding a subwoofer with a high pass filter(HSU-STF 3 SUB)for a 2 channel music system? or would i be better off looking for an upgrade in speakers overall? Currently I have large bookshelf speakers. Thank you,
I have built two 2.1 systems using M&K monitors, subwoofers and bass management controllers. I am sold on this approach to integrating a subwoofer (as well as, M&K products in general). The advantages are reducing distortion of the main speakers, reducing power requirements of the main amps and limiting bass sources in the room which can make for a better setup.

You might find this article of interest: http://www.mkprofessional.com/bass_mgmt.htm

I don't have any experience with the HSU high-pass filter or crossover, but based on the reviews of their subwoofers I'd think they'd be good.