Maybe it's because the audio out on the car sled uses 4v outputs and the RCA outputs on the player itself only are 1v.
There have been people before who said they liked the 4v sound output better. The only way to use the 4v outputs in-house is then to build a docking station with your car sled.

Or maybe the stereo system in your car is just of better quality than the one you have in-house? (so you stereo is at fault, and not your empeg?)

I'm just guessing here because -obviously- things like "sound quality" are highly subjective.