Did you also keep the 6" subs behind the seats in addition to the central sub you added?
I honestly wonder just how disappointing the OEM systems sound, even the premium one. Of course we all have different standards and expectations for such things, and what sounds good poodling about town with the roof up will sound considerably un-good at 80 top-down (if it sounds at-all...). It's one thing to demand a loud, clear soundstage with lots of detail in the highs and punchy, dry bass and another to get a shrill distorted mess with nasty rumbling lows that sound like so much farting, lol.
I'll definitely be upgrading the Professional system to Premium as a matter of principle, and I'll probably be satisfied overall. The car's engine and exhaust sound great after all...