JL Audio doesn't do birth certificates but when tested the bridged XD400/4 will deliver 2 X 280 watts into a 4-ohm load with a 14.4 volt supply. Typically a JL XD amplifier is rated conservatively enough that it will produce its 14.4 rating with a 12.5 volt supply, which is very impressive.
The Pro80s and MB-8s have the same size midbass driver with the same size voice coil in the same displacement pods. So at first glance you would think they deserve symmetrical power. However, the Pro80 will be reproducing more bandwidth which takes more power to stay clean. Also, the Pro80 has insertion losses due to the more complex internal high/lowpass passive crossover. Plus, you are going to be more sensitive to distortion at higher frequencies above that which the MB-8 will reproduce. So based on all the above, I would recommend using the bridged XD400/4 on the Pro80s and the RF 300.1 to the MB-8s.
50 Hz is too low for either 8-inch to deliver IMO. The pod displacement ultimately will determine the low frequency cut off point. To my ears, 80 Hz is a good minimum highpass number. I would be surprised if you hear more bass extension below that frequency selection. Narrowing the bandwidth cleans everything up. And, you always want to tune a system for the maximum volume used rather than the minimum.