Just one little suggestion for S&B to compensate their a bit lowish DPS, and also as an option:
Give the S&B users an additional damage which is determined by the shield you use. This damage would happen once per attack cycle OR a certain % chance, like TWF has. This could maybe increase when you pick shield mastery and Improved Shield Mastery (5/10/15% chance?). This would give a reason to use shields like +1 shock shield of bashing.
The spot for the weapon cycle could probably be the last attack or second attack.
Not sure though if it would be a bit too powerful, but of course suggestion improvements are good.