When I first started running ELs, I didn't use the durometer. This year in particular, I've had to be much more precise in my prepping program to compete and run where I want to run. I don't know if the people telling me not to use the duro were misinformed, or if the compound of the tires changed or something, but they do seem to take prep better now. I normally run over an inch of stagger, and have run 1.25" before with success. I'd say you've already hit the nail on the head as being too soft though. I've also seen that those ELs can get goofy when they get 'old' as well, even as early as 3 months after mounting.
-Andy