ehh, it depends on how able the B's are to replace them. Frankly I think Kobasew has more value in trade to another team than he has to the Bruins right now, but others seem to think he's a prototypical Bruins forward who hits, fights, battles and scores.
As for Ference, I don't think you'll get fair value for him. Unless Alexandrov is a lot better than I hear he is, I suspect he'll stay.