If you are using the .1 output from your receiver then using the single RCA is the way to go.
If your receiver doesn't have a lfe/sub out you would then consider running your left and right channel through the sub.
Try turning off the center channel in the AVR's menu to see if you're correct about little to no output from your M60's.
If they were build with 4 posts on the back make sure the bar between them is in place the connection is tight.