Thanks for posting and welcome! I'd be happy to help clear this up for you!
So the Sub In connection on the Acoustimass 300 is not actually used for anything and was removed from later models of the module as its function was never implemented in the bass module's firmware.
As the Acoustimass 300 is designed to work as an accessory speaker for Bose systems only, you won't be able to connect it to a 3rd party AV receiver. It will connect to the SoundTouch 300, Lifestyle 550/600/650, and the Soundbar 500/700.
I hope this helps! Let me know if you have any further questions.