Hooking up a sub under dash

I have boston pro 3 ways, probably going to high pass them around 50 hz, run off a jl 300/2.

i have some coax. rears run off a small 50w x2 amp crossed at 80hz.

i am going to get a jl 500/1 and a 10w7.

how can i hookup a 8w0 up front?  which rca should i use, and can i use the amps i have, it seems like i could use that 80hz down from the rear amp on it, but then i can't really do much to adjust the crossover point, as well as gain (without affecting the rears)

i could get another amp and use that and run it off the front rca out, but then i have to get a FORTH amp.  if i ran the rca from the sub rca out, then it would have the same low pass as the 10w7 which i don't want, and would be adjusted as i adjust the 10w7 which i don't want.

whats the best way to do this for ease of use and tuneability, and sound quality?  can i just maybe get a 8w3 and cross the bostons around 60 and then have the sub up front use that same amp from 60 down?  that could work but it would see a LOT of power. 

Run the rears off deck power ( they really don't do much any way except add rear fill ) and bridge the amp and run the sub at 2 ohms mono. No additional amps needed and the system will be alot easier to wire this way.

now what should i cross it at?  i wanted to do the 10w7 as low as possible to make it clean, but i don't want to miss out on midbass.  i've heard having a sub up front playing the same freq. as subs in the back leads to pathlength problems though and cancellation.  i wanted the 10w7 low-passed at 60ish, the 8w0 at 80ish and from there up the 3-ways.  can the three ways go as low as 60 ok, so that i won't need the sub up front, and would i just be better off to ditch the sub upfront?

Use the sub to it's full potential and cross it over above <120 hz and the rear subs at < 80 hz. This will over lap 10 hz and will blend the sound better. Cross the woofer of the 3 ways over at <100 hz .( again the overlap will help blend in the sound so that you won't get a definite cut off frequency and a hole in your RTA score. The subwoofer up front will highly increase your front imaging and staging so if you can.... keep it there. Up front and center bass is hard to come by.