For sure it is the supersprint backbox.
I have one myself as well on my GTA (rest of exhaust still standard) and i also notice too much boom until 2500r/min.
More noticeable inside the car, than outside. It is caused by the design of the backbox (believe because it is absorbtion demper, but not sure there - may also be reflection demper) which is notoriuos of producing these booms.
I am trying to live with it, as above 2500revs it quiets up and starts to produce glorious sound all the way up to 7000 r/min :-)
Hope this helps (and may save you the trouble of switching the center pipe, as it will not help)
Alfa Romeo 156 SW GTA 3.2 V6 24v and Alfa Romeo Giulietta 1750 TBi QV