There are two separate issues at play here:
First, is the "equalization bug" that affects all 2009-2014 RL's with navigation mentioned above.
Second, is that beginning with volume level 30, the radio stops increasing the bass and treble and increases only the midrange. This is done to reduce the possibility of distortion and/or damage to the speakers at high volume levels. The primary purpose is to reduce warranty claims due to blown speakers. Because XM (and FM to a lesser extent) have reduced sound levels compared to CD and AUX (due in part to the above-mentioned bug), you notice the low frequency roll-off function more because you have to turn the volume control up more. Ideally, a system would have enough power and gain under all listening conditions for all sources. This system does not. Better systems (including those that are considered to be under-powered) have a "feedback" system that detects distortion and limits the output to a safe level based on an actual measurements instead of blindly assuming that damaging distortion may be possible above a certain (relative) volume level.