Here is a blurb from Crutchfield's bass blocker page:
For smaller speakers (3-1/2"s, 4"x6"s, 4"s), the 600 Hz Bass Blockers work well with low-powered systems, while the 800 Hz Blockers are a better choice for high-power applications.
Speaker Size Cut-off Frequency
6-1/2", 5"x7"/6"x8" — 150 Hz
5", 5-1/4" — 300 Hz
3-1/2", 4", 4"x6" — 600 Hz (low power)
3-1/2", 4", 4"x6" — 800 Hz (high power)
I needed a pair for my Rockford Fosgate dash speakers, so I took the info that I needed to two different Radio Shacks. The first one looked but couldn't find anything like I needed and the second one sighed and said they used to sell stuff like that but don't anymore. ~get with it RS!~
I ended up buying on Ebay for $11.49 including shipping from this guy:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3062175755&category=14931
The 600 Hz blockers work great. They do exactly what I wanted them to do.
Just make sure you know what cut-off frequency you want before you buy!