Personally I'm just goimg to do the whole thing at once. But if I wanted to piecemeal it, I'd go for the res first. As far as wha tto put, that's all personal choice. I don't think there is any sort of sound tester for mufflers (but that would be very cool). Your best bet is to keep searching for old threads and hoping you find one that describes the sound of a particular pipe. The only problem with that is you'll find that almost every single one makes the car sound "a little louder", "boomier", "deeper", "not overpowering, but you can definitely hear the diff."
What does that tell you?
I'm not sure either, except that it's almost always faster

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Whatever you decide please let everyone know.