There's that and there's the difference between Tourer and saloon cars. Tourers are louder to the driver because the large boot space sitting above the backboxes acts as an amplifier.
So a saloon with identical backboxes will be quieter than a Tourer.
I used the X-powers for about a year on my Tourer but found them too loud for comfortable daily driving, mainly due to their 'drone' between 1900 and 2100 RPM, typically where the car is comfortable cruising on all that torque.
Age is also a factor. I've found that my standard exhausts - now with 242'000 km and 19 years of age - are getting 'droney' around the 1900-2100 RPM mark.
EDIT:
It would be interesting to fit louder backboxes to a R75. The whole exhaust has extra soundproofing (an additional silencer mid-way I believe) so 'loud' backboxes could be a good compromise.