I don't think you can just split the HDMI-ARC input of the TV. The soundbar can't be used with an AVR and using it for the subwoofer just makes it more complicated.
If you get an ARC audio extractor that will provide both a digital output for the soundbar and an analog output for the subwoofer. Problem is these are fixed outputs so the subwoofer level won't change when you turn the soundbar volume up and down. No real solution for this but if you set the sub level for the most common volume you play the TV if will be OK.
I would use the sub that comes with the Polk first to see if it's adequate.