i like the idea of the buff. if i had to choose another chicken breed to breed with, it would probably be the barred plymoth rock. over all of the chickens that i have rased, that breed has been the best to me. to your earlier question, it would be possible to have a bantam blood in a hevy...
Personaly, they look like to me a bantam mix with a little golden sebright in them. Over all, you have some beautiful chickens. I wish you the best of luck!