Muffler with the lowest and quietest note I've heard with the 4.0L SOHC is the Dynomax SuperTurbo. Unfortunately as koda says, V6 engines will never sound like V8's regardless of displacement, it's all about harmonics. The ST's 4.0L SOHC can actually sound quite good at idle and at light throttle. Problem is once wound up over 3000 RPM it will almost sound "ricey" the higher you go. I initially used clamps instead of welding to test several mufflers on my ST. Tried Borla, Magnaflow, and the Flowmaster 50 Series. Found the Dynomax #17733 2.5" OD the quietest with the lowest tone by far. (#17731 2.25" is stock OD) Removing the factory resonator also slightly enhanced the low tone and volume without added rasp. I did not notice added drone at highway speeds. I reused the U shaped tail pipe exiting out the side and ran it straight out the back under the bumper. It's also ridiculously inexpensive at less than $50 from Amazon shipped with a limited lifetime warranty and a 90 day trial.
http://www.dynomax.com/mufflers/super-turbo/
http://www.dynomax.com/warranty-guarantee/