Do you meen filters in general? There are Highpass filters which block frequency's below a certain frequency, which you'd use on: smaller speakers and tweeters, or you can get ones set anywhere from 20 - 30+ which you'd use with a sub in a ported box. There aer also Lowpass filters with block frequency's higher than the set frequency. Of course, the filters don't just totally get rid of all the frequency's their intended to block. They just lower the volume of the frequency, depending on how far away from the filter frequency it is. Let's say you have a 30 hz hp filter. If you play a 29 hz tone, you'll still hear it, it'll just be quieter. If you play a 25 hz tone, it'll still play, but it'll be even quiter.
A subsonic filter is a Highpass filter, it's just a low one. I have Harrison Labs highpass filters that are set at 30 hz, because my ported box is tuned at 35 hz. You can get Harrison Labs filters cheap, all they are is inline rca filters. Just plug them into your rca wires.
bass blockers are great. i have some pioneer 4 X 10's in my doors, and they were pounding so hard. they were getting way too much bass. more than they could handle. then i put some bass blockers no them, and now they sound way better and they stopped pounding so hard.