I have an observation about the 'sub-out' output. I have no way of confirming this with measurements, but judging by ear (using both test tones and music) I get the impression that the volume curve of the subwoofer output is different from that of the main speakers. I have configured the subwoofer at volume 40 and at this level it is perfectly linear (the 'sub off' setting does not change the overall feel of the bass volume compared to the cut-off at 70Hz, only its downward extension), whereas at a much lower overall volume there is too much bass and similarly, at significantly higher volume levels, there is not enough bass. In both cases by changing the subwoofer gain I can get back to linear bass.
I tested this on all digital inputs with identical results. Perhaps the fact that I am using a wireless RCA signal transmitter, similar to the svs soundpath, is relevant here.
Could someone please try to reproduce this in their 2.1 system?