since nobody mentioned it,I'll throw my .01 in here, NEW CUSTOM STRINGS. I couldn't believe the difference it made on my Cheetah and Iron Mace after putting a set of Hutch's strings on them. A lot quieter and seemed to pick up some speed. Judging only by the extra bit of thump from the arrows hitting the target harder.
Oh, and over the weekend I ditched the string stop on my Quest and just went with catwhiskers after experimenting. I left it off for now. I'm thinking that maybe the rubber stoppers available aftermarket are made of different materials. Some being louder than others. Just a thought