As far as I know, slow bulks dont give any better muscle:fat gain than normal bulks (ie 15-20% above maint.) Have a look here for more info:
Calorie Partitioning | BodyRecomposition - The Home of Lyle McDonald
When someone does a slower bulk, they gain less weight total than someone on a ''faster'' (for lack of a better word) bulk. Because they have gained less weight total, they will have gained less fat and so at the end of their slow bulk may still have some reasonable definition. What they dont realise is that while someone who used a ''faster'' bulk may have gained more fat, they will also have gained considerably more muscle.
Bottom line, I have yet to read an article that has found any connection between slightly lower caloric intake leading to improved nutrient partitioning.