I know that ProAc recommend a good amount of toe-in for their speakers, but have you tried reducing the amount of it?
While I was fine-tuning the position of my Tablette 10s I went from a tiny amount of toe-in to having the tweeter axes converge behind my head i.e. I could still see the insides of the cabinets. I was surprised that I didn’t get along so well with that. I felt the sound was noticeably brighter, and generally less well integrated. I went back to having maybe 20mm of toe-in, and I think it works better.
Before you go experimenting with cables, you might try some adjustments to both toe-in and to the distances to both the front and side walls (if that’s feasible). It may just be that your room requires some deviation from the manufacturer’s recommendations. These changes are easy to do, and may get you closer to the sound you’re looking for.