Trying to hookup my sub and from what the manual says i shouldnt use the rca cable and instead hook up the fronts directly using the sub.

The sub came with the POLK RM 6750 package

I have a onkyo 604 receiver and the subwoofer apparently has connections for FL and FR only. My question is can i just use an rca cable and then manually direct all bass management to the FL and FR speakers through menus on the receiver.

Advice please

looks like you can connect you sub up to your AVR using the L or R on the Sub connecting to your AVR's subwoofer port then run your fronts to the directly to the AVR and set them to small and as well as small for the rears..try that if i am wrong don't shoot me thats I would do it. You can use a Y cable for the left and right on the back of the sub but most here will might suggest that you dont need to Good luck hope this helps

If you use the "sub out" connection from the, you can use the Y adaptor as mentioned or simply plug the single connection into the L or R line input. One more step, turn the "low pass" knob fully clockwise.