The room (did you really expect something else from me?): it necessarily changes the amplitude and phase of a variety of frequencies. The degree to which it changes things and how degrading that change is depends on the room and speaker interaction. A cube shaped room would be one of the worst for amplitude change, as the deviations from a flat response would be huge. A "golden ratio" room (although I don't entirely subscribe to this theory), would give reasonable mode spacing and while it still changes amplitudes of certain frequencies it does so in a more uniform fashion (Bonello criteria). The other part is the phase--reflected sound is always out of phase (well almost always). Only an anechoic chamber can offer no phase shifts--but I don't think you would enjoy music in that environment. Phase shifts are what give us spatial cues.
