Aren't there fundametal problems related to the resonant frequency (although ideal speakers never resonate) of the moving parts (and the enclosure around/behind them)?You'd be surprised, there are some really good in-ear monitors on the market. Golden-eared audiophile grade good.
Really? Could you link to some?
-Glyph Reader
That is, a small speaker with a high resonant frequency will work well for playing high notes, and a large speaker with a low resonant frequency will work well for playing low notes.
Certainly in my experience, physically large drivers sound muddy, and physically small ones sound tinny.
How do you get a driver a few mm across to perform acceptably down at the 20Hz end of things?