After moving through several subs over years, I think your room modes crucially determine the SQ from the sub. I have a decent REL sub (Predator 2), and still have a massive 8db dip in the 40hz to 70hz region. The sub is unable to perform when Dirac tries to correct for this dip. My REL was visibly struggling. My conversation with few experienced FMs revealed that most subs may not correct more than 3/4 db. I then excluded this dip from the correction range for now. I will have to move around the sub in my room and see which spot needs least correction.
The takeaway : Room modes can really take away the pleasure or having a good sub output even with “best” or expensive subs. If you can, that is, your budget permits and have the space, aim for at least dual smaller subs. I have made the mistake often going for a single large sub and struggle with room modes. Better still, before you invest, if you can get to borrow a sub and experiment with moving the sub around in your room, that will give you a good sense of how room modes have an impact on sub SQ.