Have a single stack mostly out of necessity because my truck is low compression w/ reliefs at elevation which hazes some if it's not totally warmed up and I'd prefer not to haze in people's windows in the summer.
ANYWAYS, it was pretty loud with a single 72vgt, but drone wasn't horrible and you get used to it. With compound it's actually quite tame. No drone, not obnoxious at all. My biggest complain is the black soot on the cab/window after racing. I drilled a small hole at the bottom of the piping to allow water to drain so I avoid that.
My $0.02 stacks aren't worth it unless you have special circumstances (IE, because race/pull truck)