Port Passive
wiring passive sub to Sherwood Rx-4109?
in addition to either 2 or 4, 8 ohm speakers, no out on sub, no dedicated port on receiver, A/B can both be used(8 ohm min for A or B, 16 ohm min for A+B). Whats best way? Can i/should I dedicate the B speaker switch to the sub, do I use both L+R(4 wires to sub ?) Been trying to find solution for hours now
Sounds like you seriously need a new receiver. As you describe the one you have, it seems like trying to connect a sub to it would be like going fishing with a baseball bat. It's not the proper tool for the job and simply will not work.
On a receiver with A/B zones, these zones are independent and can rarely be played at the same time. Your receiver will switch from zone A to zone B. When one is in use, the other zone is off. So wiring one set of speakers to zone A and your subwoofer to zone B simply wouldn't work.
Also, to power a passive subwoofer, you need a lot of wattage and a dedicated channel on your receiver. I've never seen a Sherwood receiver with enough power dedicated to its standard speaker channels to push a subwoofer.