I would add that in placing them based on good sounding locations from the crawl, also aim for asymmetrical placement to the mains, the LP, and each other. By that, I mean different distances. This should help avoid some placement based cancellations, but can only be confirmed by testing as Shady mentioned.
When it comes to fixing those issues, inches can matter! I had a huge suck out on one channel at 50Hz, which was exactly a half wavelength from source to wall. 50Hz is about 22’, and the driver was exactly 11’ from the wall. I only moved the speaker maybe 1.5” and on my next calibration, the suck-out just turned into standard room resonance.