Yes, the comments referred to above are ones that I have read, which drove me to bring this up in this forum. I will definitely be using the HT bypass function, but not with a Denon AVR. So I’m not quite sure what to make of this. Sounds like there are many users of the M5si who have not had any HT bypass issues, but there are a few who have.
By the way, a very helpful person in the Polk Forum provided me with contact information for someone who used to work for MF and knows their equipment, has done mods, etc. He did reply and tell me that the M5si is indeed a common ground design. So it appears that I can use this with my Polk SDAs.