You definitely were on the right track by avoiding boost and cutting the peaks.
Just curious how close your speakers are placed near the walls?
I am not a total sub fiend but even though my mains go to 45Hz as measured, the two REL T9i I have provide the dynamics, impact, sustain, texture and hall ambience that make a big difference for me and my system. The RELs are more subtle and musically accurate than other subs I have tried as well as being easy to integrate and they avoid coloring the midrange- all reasons why I like them so much.