That's right--the currently available selection of headshells enables you to experiment with rigidity, characteristic resonance, effective mass (they vary in weight), and the various mechanical impedances and resonant frequencies you encounter based on materials used.
05-29-08: Pryso
There is another advantage in using an arm with interchangeable headshells. ... Raul discovered he could optimize performance of a given cartridge by experimenting with headshell rigidity and material type (resonance?), as well as weight with different model headshells.
The standard Technics HS is pretty light at 7.5g but could use better headshell leads. The Sumiko is heavier at about 12g but more rigid and has better headshell leads. You can also get headshells of various woods such as ebony, cherry, and boxwood. What happens when you put an ebony-bodied cartridge on an ebony headshell?
Then there are the headshells from Audio-Technica--their proprietary "Technihard" alloy in 11, 12, and 13g weights and also magnesium at 11g.
And Needle Doctor now offers Ortofon headshells. I don't know what thos weigh, but they look good.
As for rigidity, the Sumiko uses two alignment pins instead of one. It makes for a very rigid joint.