Thank you for your post and welcome to the Bose Community!
I would recommend what @Xyz987 has suggested and check all of your connections into your receiver and make sure its all wired up correctly.
Let me know how you get on! I'd love to help!
Jessie O - Community Support
I wonder if you have a bad cable. Maybe you could try an experiment and reverse the wire for the front left and right speaker and see if you start hearing the left speaker through the sub. Of course, that might not be possible if they all plug into a single connection to the sub. I'm just thinking that if the signal is not making it from the receiver to the sub to the cube, maybe that would explain why you hear some of it from the sub.