Just offhand i would say you have your sub plugged into the wrong connection on the back of your receiver.
You mentioned:
"I tried setting up my Marantz SR-14EX for "small" speakers on front...to send some power to the subwoofer"

Well the subwoofer has its own internal amp. You don't want to be sending it "power" from your receiver at all. Check your connections again.

"Those who preach the myths of audio are ignorant of truth."