Usually a HT receiver like this allows speaker size selections in setup. The main speaker selection may have choices like small + sub, full range only, full range + sub, etc. If you choose small + sub, everything above 100Hz should be sent to main speakers and everything below should be sent to sub.
Of course different HT receiver will have different features. If you can post the brand and model number here, I can try to find out how its setup works for you.
Of course different HT receiver will have different features. If you can post the brand and model number here, I can try to find out how its setup works for you.