The Monolith is a really good sub. I don't know about QC issues, but I don't have access to failure rate data, and no one else on the internet does either. The problem with it is that customer support for an item like this is a bit more cumbersome through Monoprice than other smaller manufacturers. So you may end up hearing about issues from it more since owners may be resorting to social media more as their support experience isn't as fluid as it would be with a less rigorous support process. We don't really know, and I don't trust social media users would be able to give the issue a fair or accurate assessment.
As for the Polk, I don't know how good it really is.
When Erin ran CEA-2010 tests on some budget subs, the Polk PSW10 outperformed all the other subs except the Elac sub, and it was the cheapest one there. My feeling is that the HTS-12 is probably very competent above 30Hz.
The Paradigm Defiance subs are pretty good in general, but the one that would have qualified for this list, the Defiance V8, would simply have been just totally dwarfed by these others. I don't think it really competes well at its MSRP. I love the Defiance X series, but I can't recommend the V8.
The Hsu and Rythmik subs are too expensive. They rightly belong in
the list from this article and shouldn't be included among subs from the present article.