A crossover unit has just one purpose--it takes the audio signal from its input and divides it into separate frequency bands that are then steered to the correct speaker for that frequency range. The differences between units boil down to how specifically this division is done. Crossovers can separate signals into feeds for front, rear and subwoofer speakers, or set to divide the signal to feed hi-, mid-, and low-frequency stereo speakers. Determine how you need the crossover to function and how easily it can do what you need it to do.
Crossover settings that are continuously variable will be the most flexible, but they do require more time for the initial adjustments. The frequency bands are divided into low-pass, high-pass and band-pass sections. Low-pass limits the signal that feeds a subwoofer, eliminating the high frequencies, which the sub can’t reproduce anyway. The high-pass section does the opposite--eliminates the low frequencies to feed tweeters. A band-pass will roll off both high and low frequencies, leaving just the one "band" that you want to feed to a speaker component. Remember, low-pass filters pass the lower frequencies while high-pass only allow the high-pass frequencies to be output.
The information in ratings that refer to dB/octave pertain to the slope of the crossover--basically, how abruptly the signal gets filtered. If this is adjustable, it might help eliminate dips or boosts in the signal at the exact crossover point.
Edit: What are you running off of your amp? Full range speakers/components or a sub?
