you should not or cannot hear any bass below 20Hz <-- all cut by THX requirements
(To ensure without distortion & sufficient output 105dB)
I believe this is the part of the reasons why SVS (not THX certified) can have deeper bass response than M&K (THX certified) under THX settings.
However, if you like deeper bass or lower frequencies below 20Hz to be delivered by your M&K MX subwoofer, you can switch the Bass EQ to "Extended". With this mode, you can have deeper bass but not to THX standards anymore.
Hope the above can provide a little bit in-sight for brothers who have the M&K MX subwoofers to adjust their settings to their preference ways.





