I had the same quandary with my 805s and whether to get the DD12 or DD15. It all boils down to room size. As it turned out the DD12 was more than sufficient for a 16 x 25 room with 12 cathedral ceilings.
My room is 15x21ft and my sweet spot is 11ft from speakers which are 9ft apart and 3ft from front wall. I have some bad modes on listening position (-10dB@50Hz / -4dB@80Hz) and other frequencies are pretty flat down to 25Hz. Yes, I tried move speakers around as well with N801D. Reverberation time is good within whole bas range (1.2sec) and I experiment with some absorbers. Still I think I need Velodyne!
It is extremely unlikely that a sub would do anything to improve a dip at 50hz. A Rives Parc or a digital corecction system would be more likely to achieve the result you are looking for. Very few rooms are flat in the low frequencies. Just look at the in-room graphs in some of the S-Phile tests and see what I mean. So you could quit obsessing and listen around/through the aberations.
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