mine works using IE7. It boils down to two things...1) the XPA-1 is a fully balanced amp. It takes the negative input and amplifies it negative and takes the positive input and amplifies it positive. I believe that means the XPA-2 uses the negative input as a reference and just amplifies the positive. 2) the XPA-1 is stable down to 2ohms where as the XPA-2 bridged can kinda-sorta pull off 4 ohms if you don't push it.
Also, the XPA-1 has better build quality and components...la de da