Any defense — 3-4, 4-3, 5-2, 1-10, whatever — is only as good as its front. The Bears were horrific on defense the past two years for multiple reasons, but chief among them was that their line was both porous and tissue-paper strong between those pores.
In 2013 the handy excuse was injuries. In 2014 the rushing yardage allowed improved from horrible to mediocre despite using second- and third-round draft picks on interior defensive linemen (Ego Ferguson, Will Sutton).
Whatever the unit or alignment, the simple mandate is for Bears linemen to win at the point of attack.