Ahhhhhh yes. The HP woofers. That makes sense; particularly at higher volumes and dynamics.
The M80 is indeed flatter over-all above the bass region. All the passive Axioms are designed with a flat listening window. In some rooms - like yours - flat sounds great. This flatness is a generally accepted standard in the industry. I'm seeing a trend in the industry though towards the declining response even for passive speakers.
It's interesting to note that while LFRs have a declining response, the active Bryston Model Ts have a flat response. I suppose this is one way of differentiating the Bryston active line from Axiom's.
Is the 880 available in an HP version?