Could have just been the stereo tricking your brain into thinking it sounded better. For example, in the late 1980's my family had a car that had an AM-only radio. One day while channel hopping one station just sounded so much better than anything we'd ever heard on this radio. Someone finally noticed a little STEREO light had lit up, because that station had recently started stereo broadcasts. Eventually we moved and later got a different car and I've never again seen a little light like that on an AM station.
