I'm not sure that is the right sub for your application HT subs don't mate well with two channel only systems
Its more challenging to incorporate a single sub into a two channel system
Low bass under 80hz will be non directional as you stated
You will need to tweek with placement and crossover settings
You don't want the sub firing back at your main speakers from across the room. This will actually cancel out bass frequencies from the sub and speakers you end up with odd bass anomalies similar to the issue you are experiencing
The best place to start is to position the sub with your main speakers firing in the same direction.
Its more challenging to incorporate a single sub into a two channel system
Low bass under 80hz will be non directional as you stated
You will need to tweek with placement and crossover settings
You don't want the sub firing back at your main speakers from across the room. This will actually cancel out bass frequencies from the sub and speakers you end up with odd bass anomalies similar to the issue you are experiencing
The best place to start is to position the sub with your main speakers firing in the same direction.