Minimum amount the speakers need to be off the wall is ~3".

As everyone has already suggested I suspect you are used to the Polks and a non linear frequency response. I would bet your old speakers had a bump in the 50-80hz range, something the M80s do not have. I had a SVS PB12-nsd that had a bump in the FR curve around 50-60hz abd it hit pretty hard, very noticeable when played with the M80s or any other speaker I tried them with. I didn't like it as much as I like its big brother the PB13 Ultra that I now own.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70