Please help me tame the bass beast

I own the Joseph Audio RM50si's which go all the way down to 20Hz. My preamp is the Classe SSP-75 and the amp is the Classe CAV-150. CD is Sony SCD 777. The 75 does not have controls to turn down the bass level. What can I do accoustically to eliminate such an annoying rumble in the mid bass. I moved them around the room but nothing seemed to really make a difference. Is the problem the amp or the speakers or the room. I have read reviews on this site and people seem to think Classe amps deliver a lot of muddy bass. The low bass frequencies seem to be okay, it is really the mid bass that is the problem. If I sit really close it is better but not great. What can I do?? My room is rectangular 18x12. Please offer me some suggestions. They are great speakers except for this problem.