There is a notion on some of these forums at times that adding a subwoofer can fix everything even if the main speaker is a weakass noodle speaker. They may ask you to add a platoon of rotund subs and it could shake the house (causing physical/mental distress), bring the roof down with one big "SLAM", etc bwaaaaaahahahaha... bwaaaaaah
But, the subwoofer is the incorrect place to look for this mid bass "slam" that you are looking for. You technically need some very legit main speakers to make that happen.
Cheapest fix could be to try optimizing position/placement of mains (turn off the sub and do this with main speakers alone running full range), alter distance between you and speakers, etc to see if the "slam" improves. But, in the end it is a limitation of your main speakers. If you are committed to your mains and insist on buying more subs, i would recommend you not buy some lardass clueless sub and try something more tactical like this midbass unit.
.LFE+Main or 'blending' it in, etc are all things you would try.