We just got through putting a complete 2.5 exhaust system consisting of a Dr. Gas Xpipe and 2.5 Spintechs with turndowns on my dads 78 T-bird with a 400m wearing some 351 4barrel cleavland heads and a roller cam conversion, haven't had it dynoed yet but it feels like a haus.
Anyhow, the spintechs actually get quieter as the rpms raise, atleast with the x-pipe anyway. I spoke to the owner of Spintech and he like so many others said that their mufflers were designed around using the x-pipe.
I've heard the X-pipe sound on mustangs before, you know that exotic car sound, with the addition on the spintechs this thing sounds wicked.
Really deep, open idle, raspy with throttle and very little resonance.