EFI Tune for Quietest Possible Exhaust Noise

I was going to say retard the timing and find and figure a way to add "muffle", either put a flange in the rear pipes so you can replace a section with some sort of resonators, or inserts into the pipes

I'm not absolutely sure, but I think maybe for "quietest" you want a resonator very close to the rear, to avoid a long piece of "echo" tail pipe