How many subs?

You may find this article helpful.  https://www.minidsp.com/applications/subwoofer-tuning/tuning-multiple-subs

There is one device that can configure them for you in the frequency domain and in the time domain with the push of a button: DSpeaker 8033, up to 4 subs. I sold mine when I bought an AVR that could configure 4 subs but regret that I did now. The DSpeaker is better. 

The answer is four.

Check this out: https://jamesromeyn.com/speakers/subwoofers/general-info-subwoofers/

Thanks for all the info.  I like the Debra system and the small magic box.

A sound guy worked in that room for hours moving 4 subs around and decided 3 was what is needed.  He used all the necessary gear to work it out.

Probably what I am liking when using only 1 sub is Rel's blending of sub with main speakers which are stand speakers.

The stand speakers are back to full range which also sounds better.  I just couldn't balance the mains with the subs when separating the frequencies sent to each.

I wouldn't use DSP if it had to involve the main speakers.  I have a very decent CD transport, musical R2R DAC and tubes at the front end of my amp.  Running that through digital and back again seems not the best idea.  But using DSP for bass?  I am using DSP - it is delays for 2 subs.  And the left sub never has gotten DSP.

The plan is to try the 3 subs again, but while considering the front left as the main one.  A combination of using Rel's preferred connection method for the left, running the stand speakers full range and less of the delayed subs might just hit the spot.

I don't know if I'd have 'gone so hard' with making a hifi room now knowing the difficulties with low frequencies.  But hey I'm here now so I need to get the best sound from it.