There just should not be a 10db swing of bass levels when the processor goes from a matrix surround mode (PL2,DTS Neo) to a digital surround mode (Dolby 5.1, DTS. I have no analog sources input to my system and levels change anytime the source changes from 2.0 to 5.1
I was able to confirm it by using an analog input, a test generator, a meter and my ears.

During initial setup, I think some of the problem might be where the 990 noise generator falls in the circuit. On my unit it appears the test generator is after the crossover and bass management so I get noise level up to the highest frequency my sub (an LFM-1) can produce. The LFM is set with its crossover to bypass but when I engage it I can hear it working. The 990 crossover was set at 80Hz. My experience with other preamps has been when the noise generator was before the bass manager I would not be able to hear any difference switching the sub crossover in and out with the sub crossover set to its highest frequency.