By "bass volume" are we talking about the sub's level? If so, it should track right along with the 1050's main volume if the 1050 is in the signal path between source and sub.
I'm concerned about one thing, and that is that your wiring is either reversed or your description of it is. The ICBM does not have a left sub input - it has a single sub input and left and right sub outputs. I think that it's just the description that is backwards. I think that in this case, the following would be the ideal configuration (following the signal from source to speakers and assuming that the source and external DAC are a two-channel source):
- source - digital transport, with digital output connected to separate DAC
- DAC - digital input from source; stereo analog output connected to left and right inputs of ICBM
- ICBM - left and right channels of DAC connected to left and right inputs; left, right, and left LFE outputs connected to 1050's 5.1 channel analog input
- Model 1050 - left, right, and sub signals from ICBM connected to left, right, and sub channels of 5.1 channel analog input; powered sub connected to sub/LFE output of 1050 and left and right speakers connected to speaker binding posts of 1050
This arrangement allows you to use bass management with a high quality stereo analog source while also maintaining a completely analog signal path downstream of the DAC. In this case, the 1050's volume control will serve to control both left and right channels as well as the sub - you can trim the sub at the ICBM once and leave it. If I've misunderstood what you are doing, let me know.
Happy holidays.