I was in a similar position as you a few years back. I have a pretty decent 7.4.2 atmos setup. I built two ultimax 18" subs with the Parts Express flatpack and the iNnuke 6000DSP, even did the neutrik connectors.
I was concerned, and skeptical, that they could really be as good as people say. After having gone through it all, let me just say this: These subs so significantly outperform every other aspect of my system that I can honestly say they're the only element of my setup that I'll probably never need or want to replace. If someone gave me $10,000 and told me to upgrade my HT, I'd use it all elsewhere and leave the subs alone.
Pioneer SC-95, 2x Crown XLS 1000 (LCR amps)
3x Epos Epic 2(LCR), 4x Emotiva UAW 6.2(Surrounds), 4x Aperion Intimus LC-I6(Atmos/DTS:X)
2x Dayton Ultimax 18" sealed(subs), Behringer Inuke 6000DSP(sub amp),
Vizio P-65C1 LED HDR
Oppo UDP-203 HD-Blu Ray Player, Oppo DV971H DVD Player, Roku Premiere+