The only advice I recall giving is that distributed subwoofers are the only practical add-on approach in the home that I am aware of that works well below 80-100 Hz. What you suggest is not aligned with this and although you might be able to get it to do something close to what you want the cost is likely to be high in terms of the quality of the living space.
It is likely to be costly financially as well, as each 600mm x 600mm membrane trap is £150 + VAT!
You mention improving decay times as if lowering them is good. Did they sound like improvements to you when you did them? What is your ideal target? Are you listening to stereo or surround sound? Do you lean more towards clean studio created music or towards reverberant concert halls? Did treating the first reflections sound like an improvement? Was this achieved for all frequencies or just the higher frequencies?
My listening is 75% music / 25% TV. I don't have a surround sound setup, only a 2.2 setup with no form of DSP other than a few basic parametric EQ filters I apply in Audirvana+ to reduce the axial modes at my listening position for music playback.
My goal by adding the additional DIY chunks to extend the coverage of the GIK TriTraps from floor to ceiling was to try to make more impression on decay times below 125Hz. In this respect the result has been positive (though not hugely so). However it has also had the effect of lowering decay times across the rest of the frequency spectrum from an average of 350ms to an average of 300ms. I was not intending this result, but I don't think it's been harmful. Would I enjoy music more with a decay time of 400ms instead of 300ms? Perhaps, perhaps not? I mainly listen to music that's been recorded in a studio environment.
I do find it interesting that I have achieved such a significant reduction in RT60 with such a modest amount of room treatment compared to what many others use. Apart from a carpet, rug and single fabric-clad reclining chair, there are no other absorbing surfaces in my listening room! Maybe I'd have been better with Corner Traps faced with reflective wooden scatter plates to retain more liveliness? (GIK didn't make these in 2013 so there wasn't a choice then, unfortunately).
What you have been doing with your room looks interesting but a bit patchy. What kept you from tackling the low frequencies?
Lack of knowledge / fear of the unknown?!
When I purchased my passive room treatment in 2013 I was a complete novice to room acoustic issues and was lead to believe that the products I was purchasing would be effective in reducing amplitude peaks and nulls and decay times all the way down to 50Hz!
My purchase consisted of eight Tri-Traps, four 244 first reflection panels, and three Monster Bass traps. The Monster traps were supposed to go on the rear wall (behind my listening seat) but I decided not to use them after I discovered through my measurements that they only marginally improved the response at some frequencies and made the response worse at others.
I have since purchased two XXLS400 subwoofers, hoping that they would both extend and smooth out my low frequency response. They've done at great job at the former but not so good at the latter, I suspect this is because I'm running my main speakers full range and have set the subs to crossover at 30Hz @ 24dB/oct which is just below where my mains roll off and is below my lowest room mode. I'm using the subs in a conventional stereo configuration sited beside the main speakers. No digital DSP or time alignment, just basic software-based parametric EQ which is applied globally to the mains and the subs.
I would consider adding two more subs if it would help smooth out the response, but I expect this will require a more complicated setup with DSP boxes etc? I'd like to keep things as simple as possible. How would I implement this without messing too much with the D/A conversion process and, ultimately, the signal that's fed to my main speakers?
I do have Dirac Live 2-channel installed on my Mac but haven't used it since switching to my latest speakers more than a year ago. Would this be of use or do I need additional hardware/software that's able to EQ and time align each sub individually? And what about my main speakers, can I keep them running full-range or will I need to high-pass them?
PS - Sorry for so many questions!