I don't have the mbox 2, but I do have the original mbox and the 002r and yes, I actually use a preamp on both of them.
I have an Mbox and I tried a Presonus Eureka with the Burr Brown 627 upgraded chips. I can say that the improvement was marginal. My thoughts are, why buy something you already have. Spend your cash on some plugins that you dont have. You can obviously get great tones out of Protools and the right plugins.
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