The trick with Multiple Subwoofers is in their placement, especially if your room is not a perfect rectangular shape. A great example is when you may have a rectangular room on 4 sides, but have a sloped ceiling like I do.
The rules of placement change significantly.
I would urge you to consider the MiniDSP option IF you like to tinker. You will no doubt be able to realize some gains in your bass response, but it will take time to get there. Doing this with the intention of getting a good response is not simply a plug-and-play proposition. IMO, it is worth the effort.
And again, pls don't misunderstand my mini-rant on Anthem.
I've heard good things about their Room Correction, too. Regardless, it is what it is.
My recommendation, getting to the heart of it, is if you want to upgrade your Sub, look at it from a 2-year plan... Buy the best Sub for your room that you can now, and start saving for a second. If you do that, you can use your old sub for a while, though it may be the weakest link in your system.
You can add that MiniDSP, and put the old sub closer to your seating position and be able to dial them in very well using REW and the measurement Mic.
Of course, this is IF you want to tinker. If you don't then make the decision to replace the one Sub now, outright, and see what comes of it. Perhaps they will work out together in a positive manner, but it would be better to have two of the same.