First and foremost, I think there's a decent chance that Jeff Green starts at the PF spot. Even if he doesn't start there, he's going to get significant minutes there. Danny Ainge isn't going to give him fairly big money to play 15 mpg behind Paul Pierce. People can gripe about it, but it's going to be a reality regardless of how you feel.
Second, I think the C's have to be hesitant to give Bass a long deal. Bass is very good at what he does--hitting open jumpers and finishing around the basket when he's set up--but he is very much a role player. How soon will it be before Sullinger and/or Johnson surpass him on the depth chart? And since none of those three are particularly well equipped to play the 5 (at least not paired with each other), Bass could very well be an overpaid bench player getting very little time in the not so distant future.
Short term it would be nice to hold onto Bass, but I'd be worried about him getting beaten out by the younger guys and then losing his value as a trade commodity as he rots on the bench.
I'd rather go with Green, Sullinger, and Johnson (with some mixing in of KG). Then I'd focus on centers like Wilcox and Krstic. That gives the C's more flexibility as a guy like Wilcox can play the 4 or 5. Both also would be more likely to agree to smaller and cheaper deals.