so if i decide to go for 4 regular 6 1/2" speakers in a common enclosure...

In theory, this would give you the equivalent of a 13" subwoofer on the low end, but with the undistorted highs that a 6.5" speaker is capable of producing. In practice, this acoustic coupling business does not perform as well as in theory -- but the effect is nonetheless noticeable. I find my dual 10's seem to go down about half an octave lower than my single free-air 10 in a previous installation would reach.

Note that you have to feed a single monaural signal to all four speakers so they act in unison in order for this to work.

tanstaafl.