I have Spintech Pro Streets with an off road X and long tubes and love the sound. Decent down low, around 2500-3000 it gets real loud and raspy under a load, and up top it makes a good amunt of noise, but sounds good!
My O/R X is quieter at idle and in the lower RPM's than my O/R H was. It also drones much less than the H pipe did. I didn't notice any loss of low end going to the X pipe, but the car does seem to pull a little bit harder on the top end. After trying both I prefer the X pipe. It's quieter driving around town but sounds great when you get on the throttle. I'm running a Mac catback and stock headers BTW.