Darryl,
Since getting good cans I've noticed that exact thing several times -- i.e., hearing a bit of extra detail on the cans that I had never heard before through my speakers, yet the next time I listen to the same piece on the speakers I can now hear the extra detail.
I'm not sure what the term for it is, but you get a pretty convincing demonstration of the phenomenon if you've ever downloaded a few tracks from a sine-wave speech site.
Now listen to the first clip again.
Here's another. Listen again in the same order -- SWS, original, SWS.
Joe