I also utilize 4 subs in a DBA/swarm configuration, with a pair of Magnepan 3.7i speakers run full range. I use my system for both stereo music and 5.4 HT surround sound audio and it performs extremely well on both as ’set and forget’ with a crossover setting of 40 Hz and a volume level of a touch under 50%.
I know for certain, through prior and extensive personal experience, that it’s virtually impossible to attain an in-room bass performance quality level anywhere near the extremely high level that a 4-sub DBA system produces from utilizing only a single sub. However, utilizing 2 subs results in a significant performance improvement than a single sub, especially at a single designated listening position, and using 3 subs can sometimes even perform close to the optimal level of 4 subs in some rooms.
From my perspective, I don’t consider this a matter of opinion but rather a matter of physics and room acoustics that has been proven and verified sufficiently, utilizing the empirical scientific method, to currently be qualified as an accepted fact.
If you’re adamant on using only a single sub, I think your best solution is to first locate it optimally in the room using the ’sub crawl method’ to avoid obvious standing waves at your listening seat, then run any room correction program the sub has. The final steps would consist of listening to as many different musical genres and tracks as feasible while making adjustments to the crossover frequency and level controls until each track sounds best to you. If you record both settings on enough tracks and genres and weight the genres you listen to the most more heavily, you should be able to plot and determine a useful average weighted setting for each control.
Of course, this won’t be ideal and is a bit of a compromise but it should be a serviceable solution until you’re ready to join the big leagues of in-room bass performance and utilize 3-4 subs in your systems.
You should also be aware, however, that even 2 subs will provide an obviously noticeable improvement over a single sub. Also, the 2nd sub doesn’t need to be identical, the same quality, size or even the same rated bass extension as the other sub to provide significantly better overall bass response than just a single sub.
Best wishes,
Tim