If you are hearing a true 20db difference, then something is probably amiss somewhere. Most rock recordings on CD are "normalized" to have extremely high sound levels (and little dynamic range) relative to other types of music (jazz, classical), and this will cause relatively large jumps in volume when switching between these recordings. My DSS video is uneven in levels between stations, but nowhere near 20db. Some channels are compressed more than others and "appear" louder, but the actual difference in level is not that much.

Beyond calibration, there is nothing I can think of specifically to check. You might narrow down a particularly loud component like a CD player or such by plugging each component one at at time into the same input of your 950 and noting any level differences.