The Mark Levinson speakers and the Mark Levinson amp, i mean the whole system, should work together to produce crisp sound with super low distortion. I don't think replacing the speakers on a non-ML system would give you ideal sound. I think probably you get better mid frequency.
I have ML system on one car and stock on another. The ML system mid and high are sooooooooo much crisper and cleaner with no distortion even at high volume. With the surround sound, listening to R&B, pop is awesome as hell.
The locations and sizes of the speakers in cars that have Mark Levinson is different than in cars that don't. You can't just put the speakers into another car without getting different door panels, parcel tray, etc.
Plus not actually having the ML amp will mean you aren't really getting the full experience anyway.
2011 GSE20 Lexus IS250 Obsidian "F-Sport 6MT" with LSD
1986 AE86 Corolla GT-S Supercharged with TRD Stuff
1993 TCR10 Previa 5MT Race Van
1991 FJ80 Land Cruiser Station Wagon
The stock Pioneer system is actually quite good, it is actually comparable to some good aftermarket set up out there. The only thing that is lacking is the low bass. Therefore, lots of members on here add JL Audio amp and bass and keep the stock Pioneer mid/high set up.