My SVT has a modified stock system.
From the cat back is stock pipes but with 4" round magnaflows at the rear positions and this weekend I installed a 40 series Flowmaster after the cat (its a very tight fit and requires some "adjusting" of the heatshield to get it to fit)
The sound is low, deep, clean and about as loud as a stock SVT system with the resonator removed. But has the interior sound of a stock system.
I would imgine that using rear mufflers with a larger case than a 4" round would maintain the low deep tone but quiet it down a little.