Seems like sometimes it's too loud and sometimes not loud enough.
This could be for a few reasons:
1. Different sound tracks will have their LFE (sub) set at different levels. Some movies (like the last Terminator) have really hot bass.
2. It could be the interaction between your room and certain low frequencies. Sometimes the interaction between your sub and the room can amplify some frequencies. If one sound track has more material in it for that frequency range, it will sound loud and/or boomy.
I agree with whippersnapper on sub setup. I originally did mine by ear and the sub was about 5-7 db hot. It took a bit to get used to the proper sub volume, but it really did sound better properly calibrated.
