Its because your "sub" isn't a sub its just another speaker. Its helping smooth response and I'm sure it is better, just not in a way you are expecting or listening for.
The frequency response of a speaker is only partly due to the driver. A lot has to do with the cabinet. There are websites and YouTube channels that explain the relationship between cabinet volume and bass response, ported and sealed, and other types of enclosures. Long story short you will be able to modify what you have or build something new and it will be better than what you're doing now but not by a lot. Probably you will be even more disappointed after all that work than you are now.
The speakers you have are pretty fine. Combined with true state of the art bass you will be beside yourself with pleasure. The tried and true way to achieve this is with a DBA. It will cost about $3k. A little more if you buy a Swarm from Audiokinesis, a little less if you DIY, which in that case depends somewhat on the finish quality you want. Mine are genuine rosewood veneer and look like this:
https://systems.audiogon.com/systems/8367The Dayton SA1000 amp is the same one Duke uses in his. Also Tim, noble100, and several of us here who have this or very similar DBA. Read the comments. This will be awesome, tight, articulate and tuneful deep bass that integrates beautifully and perfectly with your speakers. This is totally the way to go.
Parts Express has subs on sale now, with pre-cut DIY kits that make this as easy as possible and are exactly what I did. You can modify like I did to make some or all of them ported, or just build them sealed. Either way will work fine.
