M650 of the new M6-650? These are worlds apart in sound quality and the M650 has been discontinued for about 3 seasons now.
If FADE is desired, a 2 chnl amp will typically get you more wattage for less $$$. Or if there is a 3rd pair of speakers in the future, then go with a 4 chnl.
JL has always built a solid product, so cant go wrong with them. However, Rockford is not a bad line either. My go-tos are Kicker and Wet Sounds. However, it may come down to waiting for inventory to come back in or taking whats available. Ive been using some Fusion Signature series and Roswell R1 amps lately. Seem solid.
Traditional single DIN........Clarion M508.