One of the new optional rules is worth looking at here.
Even a non-thief does a little extra damage on surprise attacks, sometimes. Add +2 to damage, +4 if the target is not wearing armor.The reason for the rule? Mostly to give everyone a chance to occasionally get a Critical Blow in, to cripple or even kill an enemy. Also, though, because I thought backstab attacks should do a little more damage, to reflect being able to aim for armor weak spots. It's not much, so it shouldn't upstage thieves, but if that still worries you, just give it to thieves as well. Add the bonus after any backstab multipliers.
I had an earlier rule that surprise attacks add the descending AC to damage. I rewrote the rule this way to make it usable regardless of what armor class scheme the GM uses.
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