Monsoon technical documentation is rather hard to come by but I have spoken with several people at Delco authorized service centres. I've learned a lot, but now have even more questions than before. There aren't that many people who seem to know a great deal. Technicians at three service centres I spoke with all tell me these amplifiers are usually replaced, not repaired. This has led to few people having much direct experience, other than to look in the shop manuals.
When I went about integrating my TNR-800 to my Monsoon amp, I spoke with a manager at one centre who tells me that the Bonneville HU has what he calls a "High Level" output going into the Monsoon amp. He verified this by checking the service manual for my original HU (by its part number). I was particularly curious about this since all the GM vehicle schematics reference a "Low Level" signal. Regardless, I understand that it isn't capable of powering speakers directly. I was told that the volume is controlled by varying the signal level from the HU going into the amp.
One way to determine if the amplifier volume is controlled over the data bus it to rapidly increase or decrease the volume. If you hear a change instantaneously, it is not data bus controlled. If there is a slight delay in the volume change, the volume is controlled over the data bus. The delay is caused by the time it takes for the signal to be sent over the data bus and to be received and processed by the amp.
In your case, there may be a couple of possibiilities but I think it likely has something to do with the input level expected by the Monsoon amp. The impression I got from from the techs is that the low level output is closer to what you would see in a pre-amp output and the high level output is greater, but not enough to power a speaker directly. It'* also possible there was a large impedance mismatch between the Monsoon input and the output of your HU.
I've really been trying to track down more technical info on Monsoon amps, but still have many questions. I do know that there'* about a zillion different GM part numbers for them and no clear indication what the differences are. I believe there may be some that are Class 2, just never installed in the Bonneville.
StingRay Bike w/cards in the spokes, Weeeeeee!