I've heard the argument from modeling companies.....that flat speakers are better for models, since they're more neutral. Sounds good in theory.
Then, I plug my Vox Valvetronix head into my Bogner and Marshall cabs, which have different combinations of Celestion Vin 30, H-30, and G-75's. Guess what? REAL cabinets sound better than cheap modeling amp cabinets w/licensed flat sounding speakers!