It certainly sounds like you know what you're doing, and what more needs to be done. I know I sound like a broken record, but the only way to improve low frequencies is with more bass traps! Well, that and also optimizing speaker and listener placement. I assume you've done that already while measuring?
unfortunately I couldn't pursuit a deeper education in acoustics, at least yet. However through my degrees and a lot of web browsing, specially in your site, I could scratch the surface to at least try to do something in a pre-made room for the first time. This will be a temporary room - 2 years from now I'll be moving to a new place where I'll be able to make a design from scratch and then I will need much more knowledge than now.
I was worried about what I could see (and couldn't see) in the graphics and unfortunately I still don't understand some of them. I totally understand you now when you say that there's never enough bass traps.
I took some more measurements yesterday after tilting the monitors slightly up. The SPL and Waterfall remained mostly the same.
After that, I decided to pull the microphone a bit further away from the speakers because that was where I was listening some difference in the low end. In this position, the mic is basically outside the equilateral triangle instead of inside. What happened is that the 55-60 Hz null I had, completely disappeared on the right speaker (which was the worst on the regular listening position) and it was much bigger in the left speaker (which was the least noticeable before).
From what you can see on the graphics I previously posted, do you think the remaining problems are something to be worried about, or should I just move on for now and place some more traps when I have the time ? I'm thinking of straddling the wall-floor behind the speakers and the wall-floor on both my left and right side, that's where I have spare room.
Hi kraft. I realize you're not asking me but, from what I understand, adding a subwoofer can be useful in that if you place it in an area of the room where there might be a peak at 60Hz, it can help balance the response of your speakers at the listening position.
A sub can help, but at some point you just accept that what you have is vastly better than before you added bass traps, and not fret over the problems that remain. I have a lot of treatment in my living room, and it's still not perfectly flat. But it sounds awesome. The difference between having four bass nulls all 20+ dB deep and one null only 5 dB deep is huge.