Weird

Just did some work on my Forte 4s. After removing the bi-wire binding post assembly, I discovered that one bass hook-up pair was wired out-of-phase at the factory. Hadn't noticed possible because my subwoofer (single) located directly behind this speaker.  

Here’s a possibility:  From the manual of my preamp

“THE LINE STAGE OF THE PV11 INVERTS PHASE OF ALL INPUTS

(including phono). If your system has an odd number of inversions, then

you must add one phase inversion. THIS IS CONVENIENTLY DONE BY

REVERSING THE POSITIVE AND NEGATIVE CONNECTIONS TO

YOUR SPEAKERS (be sure to reverse both channels).”

I have reversed the positive and negative as indicated. 
I wonder if the subwoofers are affected by this.

I doubt that the inversion of absolute phase has anything to do with your problem. What strikes me as possible though is that with both subs running perhaps you are just getting too much bass and this excess bass is muddying up your upperbass/lower mid range. You have a bass control on the back of each speaker. Play with this for a while, starting out with minimum bass on each speaker and increasing it until the bass can be heard/felt - you might do this one speaker at a time - perhaps because of your set up/room interface only one speakers location is problematic. From your pictures nothing is obvious re room acoustics, etc. (Except they appear to be on the long wall - in my experience speakers usually sound better on a short wall firing down the long dimension of the room. (Think shoe box type concert hall.) 

Maybe plug it back in and then unplug the other one?
You’ll have an extra data point at least…