Man... sounds like you are in a conundrum. Although I have an appreciation for well executed low-tech tanks, high-tech offers just more interaction and excitement. I have a few low-tech tanks which can only exist because I have a few high-tech going...LOL BUT, IMO, ADA really shines in a high-tech state. Just something about the name and sexy rimless that go hand-in-hand.
Remind me again. If ferts are on point and co2, then it may be the bulbs. Did you swap those out to giesemann midday and/or powerchrome aquaflora? You can have a crap load of high PAR, but if spectrum is off from old bulbs or low quality ones, the plants just tend to fade to green, IME... take the Finnex Ray 2 for instance. Loads of PAR, but lots of complaints of fading red plants... that is until spectrum is supplemented with a Monster Ray or changes to the fixtures, as with what is now being offered in the Planted+.
I have not replaced the bulbs yet. With the holidays coming up, trying to save every penny as this year will be expensive. If I change out all 4, it would be pretty costly. I may try to do two bulbs and then I would keep my 6,7000 and rosette bulb. We will see.
Trust me, I feel you on ADA and high tech. I was thinking, if I go low tech, I may as well get rid of my inline heater, lily pipes, filter and just use a HOB and an intank heater, haha.
I was also thinking, in an idea world, I could just swap the fixture from my holding tank over to the 60P. I just gave away another one that had good bulbs, I actually have a 3rd with no bulbs. The cost to replace the bulbs is more than I want to spend for an experiment, especially considering it's a 30 inch fixture I wouldn't use even if it worked. Then I realized I do have two 10 gallon hoods I could use.
So, I may swap the fixture over soon and just test the differences for a week. I do have a lot of algae so it may not be a good test. I could keep the CO2 going but I think that may help with algae, but not with color.
I am not sure my ferts are on point either. I have been doing EI but I haven't had the best of luck getting color with EI. In the past, when I have messed around, I usually did more harm than good.
Just for fun, here is a terrible picture of the holding tank. This Rotala was about the same color as HM before I moved it (2 days ago). I feel the color portrayed is impressive going to much lower light for 2 days but it's even more red than it shows. It's not just the top growth either and the light is higher than the rim of the tank, actually almost the same height as it would be if on the rim of my 60P.