Is this with the M80s large or small crossed at 80hz? Where is your sub in the room and relative to your mains. Don't worry about the low end sub response, the 500 has a brickwall filter at 19hz which is why there is sudden output.

Overall there is room for improvement. You are running the 500 very hot compared to the M80s. One quick suggestion: raise your sub trim adjustment to full, which would increase output by 6db over 33hz then lower overall sub volume by 10db. That should blend the sub/sats better and give you smoother response below the crossover point. Also try reversing phase to see if that helps smooth response.

John