And yes it sounds better!
I am glad you managed to get the integration with the main speakers more or less alright. I am in a somwhat similar situation with just two subs (once one of them is back from repairs) in a large (some 7000 cubic feet) open plan listening space. So I thought about what you could do next, with or without spending some serious money. These would be my steps:
1 get Multi Sub Optimizer, for better subwoofer equalization than your AV receiver can do. You can use the filter suggestions as input for your SVS subs. This will cost you nothing other than time.
2 The same, but now add a minidsp 2x4 HD to allow more, and more sophisticated, filters. You wrote that you do not want to use the 2x4HD because it did not measure very well, but if you only use it for the subwoofers this will not be audible. This will still be a cheap upgrade with probably quite audible improvements.
3 Add your two cheap existing subwoofers if you still have them. They will not add low end extension, but they may smoothen the response a bit, and over a wider listening area. See here for inspiration:
http://archimago.blogspot.com/2020/05/musings-measurements-subwoofers-to.html
4 Either, and here it gets expensive, expand on this idea with two additional SB3000 subwoofers. This will give you quite a bit of additional power, and a smoother response over a rather wider listening arrea.
5 Or use a similar amount of money for just a third but this time really big subwoofer, preferably located in a corner for additional reinforcement. This is the one advocated by Earl Geddes. It will give you a lot of really low output, and a somewhat smoother response over a wider listening area, though not as smooth and wide as with a total of four subs.