i have a apex ws with the resonator cat and the diffence in sound is very noticable than with a stock cat, especially when vtec kicks in. did your cat come with a new header to cat aluminum gasket thingy or did you just use the old one. i got a new one and made sure that i connected it tight and got a good seal. theres no leaks that i know of but it is louder especially at wot.

another thing you can try is have a friend rev the engine while you listen near the cat and again at the ehaust tip. if the sound is comming from the tip then you have your answer. gl.