There is no way to directly integrate a 3rd party subwoofer with PLAYBAR. If you use AMP for the rear speakers, you could use the Boston Acoustics unit in the rear.

I was under the impression the Amp’s subwoofer output was turned off when being used to drive surround speakers?

You are correct: https://support.sonos.com/s/article/4717?language=en_US

SUB cannot be used with AMP when in the surround configuration.

I don't run a non sonos wireless sub (have 2 sonos gen 3 subs). But I do use an audio out on the tv to send audio to an amp (which filters to 30hz) which then drives a pair of aurasound bass shakers. No reason the same couldn't work for a regular sub. 

Thanks everyone for your answers and insights. I think you all got the question correct, even though I made a mistake in the body of the post referring to my wireless subwoofer (correctly) as the wireless soundbar (incorrectly). I guess I’ll just let the BA system keep collecting dust. It really sucks how quickly technology makes these kinds of things obsolete. Back in the day, you’d keep your speakers for 20 years, and now you’re lucky to have them for 5. This is one of the reasons why I’m hesitant to buy any more Sonos.

Yes. I have a perfectly functional cellphone. Unfortunately, the carrier has informed me that the phone is too old and soon the carrier will stop forwarding calls to the phone. I can’t even re-purpose the phone as a remote control because the Apps that I’d need to use will not run on such an old phone. And the web browser will not load some of the latest websites.