What you were told may be part of the issue. Muddy bass can be caused by many things including your amp, speaker position (lower bass null or doubling modes caused by your room), cables, ancillary gear, etc. As a general rule of thumb, the speaker panels should be at least four feet from the back wall. I found, after having owned four different ML models including the Summits for four years, that five feet seems the optimal position.
Roberto's point is that the two bass control knobs should help address the issue. Try this. Set both knobs at -10db and listen. The sound should be anemic, dry, and thin without any body or tonal saturation. One example of this is a female voice recording that will have excessive sibilance and no ambience. Another is the string section in a good classical music recording which will sound thin with inaudible decay. If it's still boomy at these max. attenuation settings, look elsewhere to solve the issue.