If you have a pdf of that Cox paper I'd like a copy..
I would agree that the bass traps will give more noticeable benefits than diffusers and seeing as we are dealing with JBL's here...you gotta get the bottom end right!
You can half solve your rear diffusion issues by careful placement/filling of those bookshelves.
Have you got access to any gear for measuring the changes. It isn't that expensive to get into it really compared to 15 years ago. EASERA is a great bit of software and can be run off a moderately priced soundcard. We use some nice Bruel & Kjaer mics but a £100 Behringer mic is more than adequate for this type of stuff.