Finally onto something - I was able to get the distortion go to away when I removed the connection to the LINE OUT jacks on the M5si.
The LINE OUT was going to an NAD integrated amp that I use for some in-wall speakers in another room. The NAD was in 'standby' mode when I was doing my testing, but just having the M5's LINE OUT connected to the NAD was causing an issue.
I tried connecting the M5's LINE OUT to an input pair on my AVR (also just in 'standby') instead of the NAD, and had the same issue. Also tried another pair of RCA interconnects and the problem remained.
So it seems that the M5 is somehow adversely affected when its LINE OUT is connected to an input on another component. Thoughts? Anything else I should check out next?
Thank you,
Greg