The idea of placing one of the mains on the floor where you want to place the subwoofer is a good one. You could also measure your mains individually and see if the null is present in both speakers, and then make sure to move the one that isn't producing the null to the future subwoofer position.
You could also experiment with moving the speakers further/closer to the wall to see how this affects the null.
Finally as you indicate, you should ideally have a crossover higher than the null, so probably 100hz. It also sounds like you listen quite nearfield here, so even if you do have some dips, it's not necessarily very audible. Could you perhaps share a screenshot of your measurements? Even very deep dips like the one you indicate having may not be that problematic if it's narrow. One or two narrow but deep peaks somewhere in the 80-100hz area and above is very common, and not always easily fixed.
Hi, here are my measurements without any dsp (only built in dip switches engaged with the genelec 8030C)
background, both speakers are right beside my computer monitor, both placed equal distance from wall with tape measurement (close corner 5cm, far corner 7.5cm) toeing to me.
The environment difference is that the Right speaker right side is basically free, ~ 70cm from window, but left speaker side are packed with small gadgets for my wife (files, Xmas cards, small wooden jewelry box etc. literally touching the left side of my left speaker, desk space in front of both speaker is around 40cm, with another 50cm behind being my listening position and right behind is a bookshelf with panel door.
I could see that the Right channel have the dip further at 100hz and left channel approximately at 85hz, so probably it is related to the front bookshelf door reflection being ~1m to both speaker axis causing the 1/4 wavelength cancellation. while the left channel maybe those small stuffs of my wife have been acting as some sort of bass trap so the overall bass level is lowered quite significantly.
my only space left for sub would be right at the middle of my desk in front of my listening position, on right side is my computer tower case which almost touch the bottom of my desk, and left side a bit more free, while if so using sigber or genelec 7040 would make the sub driver to bookshelf (ignore my chair and legs) approximately 80cm so I assume would at least making the null at around 100-110hz, so likely it could make flat bass till 20hz and the null would be kept at around 100hz?
L: Green, R: Red
L+R
