After living with the 4 subs for a short time, it is clear that more really is better!
The bass line, along with LFE in soundtracks, is so smooooooth. All enveloping, each seat now gets the full impact. I thought my dual sub setup was good before, but now I am blown away with the impact of 4 subs.
Don't know what took me so long to make the leap, but I'm glad I did.
The difference is night-and-day!