Personally, I tried a cab w/ blockers and heard the "phase-y-ness" and did not like it.
What worked for me, as far as diminishing the high end "beaming", was really, really breaking in the speakers well.
I did try with the foam donut thing and that seemed to work but I did not play with it long enough. It was impractical for me because of the constant travel and popping off the grill for close mic-ing.
Your mileage may differ. Through physics the "why" can be explained, but in the end it's all subjective to what you, your FOH engineer, and what the audience is perceiving.