PSW108 hookup to a receiver with a single speaker level subwoofer output

Hey all

Bought the PSW108 as a boxing day special in part to motivate me to hook up the surround system again.

I have a Panasonic SA-HT400 receiver, it's a pretty decent unit that originally came with it's own speakers, including a subwoofer.

The subwoofer was passive, and had it's own mono speaker level output connection on the amp.

Trying to figure out how to hook the PSW108 up.

The manual indicates using a line level, which I don't have; Or to use the front channel connections, which I dont want to do, partly because I want the sub at the rear of the room (I don't want to run double the speakerwire!), and partly because I'm guessing that this unit being a "theatre in a box" has the low frequencies already filtered out of the front channel connections.

I don't want to return the sub and being post christmas, buying a new receiver is currently out of the question (but something I may do in the next year).

I was thinking of getting a speaker level to line level converter and just run the one sub channel through it and using the line level in on the PSW108 - or - running speaker wire from the receiver to the sub's speaker level input, bridging the L/R pair.

Thoughts?

Thanks in advance!