Connecting Mono Speaker-level Output to Stereo Speaker-level input on Active Subwoofer

I just replaced an old Sony 5.1 (with pre-out subwoofer) with an Onkyo HT-290 (with speaker-level mono subwoofer output). My subwoofer is a PSB Stratus Subsonic 7. This active subwoofer has stereo line-level inputs AND stereo speaker-level inputs. I need to connect my mono speaker-level output from the receiver to the stereo speaker-level input of the sub.

The PSB manual does not have any diagrams to represent this configuration. Nor does the receiver since it was meant to be used with a bundled passive sub.

I have stripped a couple inches off the end of the subwoofer speaker wire and connected the positive terminals and negative terminals in parallel (using their respective wires). Is this a valid connection?

Having a hard time finding impedance information for this sub. But, it may be a 4 ohm sub. So, I may be better off with a series connection?

JHaz, Bill,

Thanks for the replies.

I had connected to both terminals because that was the recommendation in the manual when using the line-level inputs. I just finished calibrating the speakers and was happy to see the sub performing as well as it did with the previous receiver (as far as the sound meter and calibration tests are concerned).

I'll go into one terminal and observe how that sounds.

My assumption is I'm still better off with my PSB active sub than the bundled passive sub. It hasn't even been unboxed.