Roly suffered from the same problem when he was a pup, and any type of dog treat biscuits didn't help at all. He was worst on Burns, so then we switched to JWB Junior fish and he was much better on this (and not treats), but when we went onto the adult version it started all over again. I switched him onto Chudleys and he was good on that, but was conscious that it wasn't the best food around and eventually switched him to Arden Grange Lamb & Rice - haven't looked back. Poos are always firm and we have been able to introduce a few treats as well without any ill effects. I think partly that he has grown out of it, but occassionaly he will have a treat that will upset things - Bonios are definite no for Roly.
I would suggest Arden Grange.