How is your latest work? I looked over the thread and agree with the last few posts. I think you initially were trying to push the OEM underseats too low.
It is normal for the center to seem quite strong with Logic7 initially, you will learn to love it, of course since you haven't installed the R4's in the doors yet the center is going to be the most efficient driver in the car. However, the MS-8 should account for that.
I'm still getting my own system dialed in, finding better results at the -26 to -28dB range (amped) I think -20 to -25 should work for you but try all the way to -30 if it helps. If you notice "scratchy" distortion when turned up, you probably calibrated at too LOW a volume.