I think 60Hz and 80Hz is ultimately where the original poster John (three Johns in one thread!) will land, but for trying to detect the problem, I think going with 80Hz all around ground is best.
Just going from the posted curves (and I'm nowhere near an expert in interpreting audio curves), I...
In my opinion your problem is setting your crossovers too low.
I have the SVS Ultra Towers as well as the Ultra Centre (I use 4x SVS Prima Towers for my rears and surrounds, and 4x SVS Prime Elevations for my height speakers), so my front stage is exactly the same as yours (though I have 2x SVS...