I have the MAC Boom Tube on an otherwise stock 06GT. I run them with and without the factory Cats.
With Cats - Nice volume, deep tone, some resonance but not bad. Mostly around 60 or 65mph, I usually run 45-50 in town and 70 to 85 on the highway. At those speed resonance is not noticable much and the sound level is good. There but not intruding. Can still speak in the car. But the cats kill the "pulses" in the exhaust so it sounds "flat", no cylinder pulses. That seems to be a factory cat issue though.
Without cats (H pipe) - Loud! Cylinder pulse are there and it sound nice. Driven under light throttle is is perfect except at around 60mph, can't hold a conversation in the car. Not a problem for me, not many people ride in it, and I spend little time at that speed. Medium throttle - "almost" too loud. but awesome. Full throttle, well.... my friend called me from over a mile away and said was that you that just ripped thru 3 gears? Ummm yea, where are you? At home. OMG, I just did a u-turn on the interstate and got right back up to 80. So yes, distinctive (and cool) but LOUD!
Now I am almost 50, but... l love a good v8 sound, and the loud was proven by the friend calling from well over a mile away.
I'm thinking they may sound perfect through hi flow cats.
