Guys took the words out of my mouth, if you can swing a W6 that will be the cleanest sounding sub out of what has been discussed here. The W6 is JL's flagship sound quality sub, the W7 is simply a compromise between output and clarity.
The Type R (or Type ARGHH) is a classic bass head sub, they are known to soak up loads of power beyond their ratings and flat out pressurize a cabin for cheap money, but not known for their clarity, and also I believe much deeper mounting depth than a W3 or W6.
A kicker L7 will have a rather unique sound, the square shape allows for greater cone area which increases output per wattage, but also non circular cones will color the sound, and muddy the audible reproduction. Not as critical on a subwoofer as on a speaker, but still a step away from clear/clean sound.
Whatever you pick a proper box will make the best out of the driver, then feed it all the wattage it can safely handle and spend some time tuning the system and you should be happy.
I am waiting to see how the new SI bM mk4's are goin to perform, I plan on getting a couple once they ever come out lol if you can find a good bm mk3 or 2 somewhere I would do that, good shallow mount.. In my 2 cent opinion
I like the way you think, I still dream of 4 BM's behind the back seat :redspotdance:
But I love my Mag V4 in the center console, super clean and pretty sure I will never get rid of it, just kick myself since I had 4 in my possesion at one point and now only 1.