hi im spaceg, im new here and i have an 03 sierra 4.8L with summit truck headers, shorty style. i took a 70 series flowmaster to a local truck shop and had em cut out the gm mufler and weld the flowmaster in its place. the factory cats are still in the y pipe and i have a single pipe, 2 1/2" i think going in the muffler, and a single pipe coming out to the tip
the truck is somewhat quiet at idle. you can hear a low end rumble in the cab and outside the truck. it is about the same level. im not sure if the headers make it sound so clean but it sounds clean. at the tailpipe, it is distinct. at twenty or thirty feet away, the rumble is the same, very nice, but not a pain in the ass for my neighbors.
on take off, its agressive, but more subdued than a 44. the low end rumble is about the same as the 44, the extra length of the 3 chambered 70 cuts down on some of the higher growling, but more muffler case surface helps the low end. point being, it is very flowmaster, and its a joy to drive.
its been on for one year and 20,000 miles, its gotten better with time. long trips from MD to NC and the beach and mountains have tuned it in. when i go in the house i can hear it in my head for the rest of the evening.
the 50 series sounds like a nice middle ground between the 44 and the 70 but thats only in print. ive not heard myself. however if you are planning on duals, the 44s or two 50s may be nice.
if you are like me and you want the drive to be awesome for you and at least let people you are driving a machine when you go by, the 70 is a great muffler.
hope it helps. enjoy the fantastic drive home when you get er done!