Bringin' back the high school memories now....
Yes they work, but you've got to pay attention to your cross-over slopes.
The frequency you want depends on how low of a frequency you want your 6x9's to try to play. 6x9's can actually be pretty good with low frequencies and proper amplification. If you upgraded the 6x9's to aftermarket ones, the documentation (or the box) ought to list the low-end of the frequency range. I'd match the bass blocker up as best you can to that range.
For 6x9's, that 0-150 one from Crutchfield sounds good to me. The other ones are more meant for smaller speakers (4.5", 3", 1" tweeters) that can't handle low frequencies at all in a setup where larger speakers (5.25", 6", 6.5",6x9) to handle the midrange.
However! I'd not use them at all. Crossover points & slopes are critical in getting good sound in home and car audio. I'd not trust a $2 part to do that properly. I'd get an actual active crossover or a little amp with crossover controls to do the job. Something so that you can precisely split the frequencies going to the 6x9's and the sub. The 6x9s would sound much better with an amp anyway, rather than being run off the headunit.
Does the sub itself have a crossover? I know I had one once (back in the day...) that had rca & line-level inputs, and rca & line-level outputs controlled by the sub's variable crossover. It was handy, because I could just run the rear channel directly from the headunit to the sub, dial in the crossover I wanted, and then the rca-output from the sub to an amp running full-range (already crossed via the sub) to power the rear speakers (Polk 6x9's, actually - loved em). Made it very easy to dial in the crossover correctly.
I wouldn't think you'd want your sub to try to produce much over 100-150hz. Anything higher than that and you're going to start to get 'directional' sounds from the trunk. Not so good.
Installing that 0-300 on your rear 6x9's would leave you with a lovely 'Bose-esque' hole from 100-300 hz.
Good luck and have fun. I sort of miss the days of spending hours in the garage after school tinkering on my car's stereo. Fun times.
