Will CD or Preamp to drive Headphones?

I have read that the output impedance of most cd players is on the order of a few hundred ohms. If my headphones have a similar impedence rating, is there any reason I can't simply connect them via an adapter to the RCA output jacks on my CDplayer? Or, if using a cd that doesn't have a digital volume control, what about connecting the headphones directly to the variable outs of the Preamp? would this be different for a tube preamp as opposed to a ss one? There must be a reason why people (including myself) buy separate headphone amps... On the flip side, I have used my headphone amp (earmax) as a preamp - the sound was very bright with seemingly exaggerated bass. I would appreciate it if someone coud shed some light on the technical differences betweeen Preamp and headphone amp output and the feasibility and/or risks of interchanging them.Thanks!
only guessing, but the input impedance of components is HIGH and the output LOW so the thing doing the 'driving' doesn't blow itself up with too much current flow. You might get away with driving earphones with a component NOT designed for it but it would be risky and might damage the driver component. I think the reason the Headroom sounds bad as a preamp is the same reason.... wrong impedance matching: The Headroom is to drive a (relatively) low impedance at (relatively) high current and a normal preamp is setup to drive a high impedance, lower current component. (anyone wanting to bash me for this explaination.. feel free to do so... I am just extrapolating from reading stuff...)
Don't know, but I suspect you are correct about wanting to match impedances. But when I was reading the review for the Musical Fidelity A3CR preamp, the reviewer was lamenting the fact that it wasn't set up with a headphone amp, as it (paraphrasing) should have been adequate, what with roughly 3 Watts of gain (going from memory, so feel free to correct). So sometimes they can, but I wouldn't try it unless I knew what I was doing...