adding a sub to my definitive bp 7002's ?

although my definitive 7002 have 12" subs built in, I feel as if they don't go low enough in ht, I am considering a 15" sub such as a hsu, svs or rythmic to ad to my system, looking for opinions on this, thanks
the Def Tech subs are better than any of those you mention and match your mains very well, and yes I have heard them all, and owned and loved the SVS Ultra's very much.

I am shocked you need more bass? Could be the room, the Def Tech 7002's go VERY low. When I had the 7004's in a home theater, the Velodyne DD 15 measured them down to 22 hrz when I set them up together. I moved the Velodyne to a 2 channel system, because it was really unessisary in that system.
I owned the 7002's and later the 7000s. I had issues with getting their best of low end power and weight. In the end, I chalked it up to 2 things, the room and lack of synergy with the amp connected to the speakers. I would look at experimenting with other receivers or amps. The 7002's should go low enough if all the bass signal is energizing those subwoofers.

The Def Tech subs by the way have a very good reputation. I had HSU subs and over time I eventually sold them.
Adding a sub makes perfect sense and any of the ones you mention will be a benefit. The issue with subs built into speakers is that being physically connected to the rest of the speaker, they can not be placed appropriately for good bass response.

Regardless of the sub you choose, you should also consider room mode correction if you're not already doing that.