This is a great speaker project for smaller rooms where a smaller speaker is required, and deep bass isn't required.  This is a fairly straightforward vented design.  The woofer is a 5-1/4" unit from Radio Shack, which I believe is still available.  I had the speaker sitting around for about 8 years.  It is intended to be used as a full range speaker (thus the dorky whizzer cone), but when crossed over, it actually has excellent bass extension and a very clear midrange.  The tweeter is a 1" silk dome unit by Sound King.  They were available from Speaker City for a while at $14.00/pr.  They are ferrofluid cooled and have a fair power rating - certainly enough to keep up with the woofers.  The crossover is built in to the terminal cup.  It is an Infinity unit from Speaker City, which I presume they got as a surplus item.  They use a 2nd order design for the both high and low pass sections, crossing over at 2500 Hz at 8 ohms.  The components aren't the highest quality, but only a very discriminating ear will hear the imperfections.  The nice feature about these crossovers is that they include an L-pad attenuator for the tweeter, which allows for better matching between the tweeter and woofer.

These are nice sounding bookshelf speakers.  In a smaller room, they provide a very satisfying sound.  They are particularly appreciated in a live room where larger speaker would provide harsh and boomy sound.  The Bk5.1s produce a very natural sound with excellent human voice reproduction.  A very musical speaker.  They will play down to 50 Hz and below, but lower bass is attenuated.  With a reasonable power handling, they can play fairly loud in a small to medium sized room.  They are well suited to surround applications, main speakers, or in conjunction with a powered subwoofer.