2-channel receiver, 3-speaker wiring?

[Flash21]

I have a bedroom system consisting of an old but nice Denon DRA-400 2-channel receiver with a pair of Cambridge minimonitors and a small subwoofer (non-powered). I am trying to figure out the best way to wire this system.

All 3 speakers have simple L/R connections, with no system crossovers. Right now I have the wiring for one of the monitors running through the subwoofer, which actually seems to work OK...but I am uncomfortable with what I assume is an asymmetric L/R impedance load for the receiver. Should the subwoofer instead be run off of the B speaker outputs somehow? Can I combine the B-speaker L and R wires at the subwoofer to get bass inputs from both channels?

Even though I have been in audio for 25 years I am a bit hesitant here as this is my first 3-speaker setup...I have run searches on this topic but am swamped by car audio and home theater questions.

[Flash21]

I tried running the sub off of the B speakers, with both L and R hooked up to the sub. It did OK, but the receiver seemed to get quite hot... not sure if this is unusual or not as I haven't run it that much last few years, but it seemed hotter quicker than I expected...I could just keep my hand on the cover over the heat sinks. This suggests that the above wiring idea might not be the best...any other suggestions?