I have the RSL 12S, HSU ULS-15 (actually 2 of each) and a flat pack 18” sealed sub I built on my own. The RSL is a fabulous sub. I prefer it over the ULS-15 if I had to choose between them.
If I was starting over from scratch, I would build my subs. My biggest regret is not jumping into the sub building arena earlier. It seemed intimidating. It’s not only easier than anticipated, but actually gratifying. An 18 inch driver provides the impact I was missing and wanted, and came out to around $1K for the sub build + amp. The sealed sub I built is close to the same size as the RSL 12S.