Sheb, a difference in SPL is a difference in volume. If by "jump" you meant a notable volume variance, then by definition the speaker/sub levels aren't properly calibrated. If you've been attempting to do this by ear to "taste", it wouldn't be surprising; our ears are great for enjoying music, but for measuring they suck.
Proper calibration can best be done with one of the auto-calibration systems many modern HT receivers have. A second possibility would be using an SPL meter such as the digital RadioShack unit. The most common problem is setting the sub level too loud. The sub shouldn't be heard as a separate entity, but rather that it isn't there and the speakers are putting out more bass. I'd hazard a guess that the point where you say that you "lose volume" is closer to the correct one.
