I'm not a fan of the "axle back/cat back" mufflers. They hang too low and speed bumps, dips, steep grade driveways and offroad trips tend to tear them up. Otherwise, the sound is great.
I had a 99 Outback I just removed the OE muffler and installed a 18" bullet glass pack that remained tucked under the car with a chrome tip. Sounded great on throttle and cruising and no chance of contacting anything.
Watch the ground clearance and keep an eye on the heat distribution on the bumper cover.