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.

I have gotten further.

Front left - Rel high input using L and R as the main sub with stand speakers full-range - as I said I would do. The sub against the right wall (I think 3 ms delay), being nearly opposite the main speakers wasn’t doing it for me. Only having the back right (9 ms delay and 180) seems to work well with the front left.

I can hear ’a hole’ in the back left in bass. But it is a difficult room. 14 feet by 12.5 feet (that being the front wall) by 11 feet. The difficulty is a diagonal in the back left corner, amongst other standard difficulties.

I haven’t been using a low pass filter that is necessary as the 120Hz 4th order low pass on the Rel theatre inputs aren’t enough to remove higher frequencies. I want to sell a piece of expensive equipment that could take care of this. I know the driverack can create an appropriate low pass and I am hoping it can combine L & R to 2 mono outputs so I can properly test the 3 sub design. I am waiting on someone who can program it.  For the moment R is the input for it.

I can easily change it from L & R to all subs to L to the left sub and R to the right sub(s) by disconnecting a wire and removing a cable - once the driverack can accept L and R and make it mono.  I could buy a Radial Mix, but as I just sold 2 of those, I prefer the driverack to be able to do this.