It was likely that the IB2 (non i-spec) was also of the old cabinet material and build process. The new material and process, which was integrated as an i-series upgrade features Medite instead of MDF, and increased cabinet rigidity. So with a more rigid cabinet you also get more defined LF performance. All in all the i-series upgrade is a whole lot more than just a new "$100" tweeter and sheet of veneer on the back.
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PMC, Bryston, Purist Audio Design.
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