I have a pair of radio shack 15's (yeah lolol whatever...) in a sealed Atrend prefab enclosure, in side the trunk of my car.
Even when the EQ is flat, no bass boost, etc, there are songs I listen to that have sine wave bass notes (southern rap music) that are no where near as loud as the real lows and highs. I see the waves on the oscilliscope in the windows media player and they are heavily amplified like other bass notes within the track. So this is not a case in which certain bass notes were recorded softer than others.
If not that, then what may be the cause of this loss of SPL?