According to the website section for the RC (you are looking to change the speakers in the RC-F not an ML?)
"An eco-friendly technology initiative, GreenEdge™ improves sonic performance while reducing power consumption, weight, and heat generation."
It also states -
"Mark Levinson engineers have spent thousands of man-hours collaborating with Lexus engineers to achieve the precise Mark Levinson Sonic Signature for each vehicle. That means determining the optimal system architecture and placing speakers so that each passenger hears the full concert—a three-dimensional orchestra from the front to the back of the car."
Clearly if their solution doesn't fit with your ears you should embark on a journey of experimentation.
I love auditioning source/pre-amp/power amp/speakers, usually in store first to narrow down the selection, then at home to hear how it interacts with my room. Hours of fun. Shame the number of retailers willing to indulge is rapidly declining ...