cat55

I just replaced my 1997 civic Si catalytic converter and muffler front end at a muffler shop, using an aftermarket CAT. I went back today because it was making a god awful noise, and was told it was because the CAT they put in wasn't compatible with the Honda muffler back end and the noise was caused by reverberation. They said they're fix it but since they told me the aftermarket part was compatible yesterday, and that it was not today I'm not sure if I trust them to make the best choice (as opposed to the cheapest and most convenient for them). So, I'm not sure what the best solution would be:

1) ask them to replace the aftermarket part with an OEM part so it was compatible
2) ask them to make sure that the aftermarket part was compatible for the Si as I have had to order special parts before
3) Let them replace the part of the muffler that is still a Honda part (now about 10 years old) with a new (but probably crappy) new generic back end of the muffler.

Any opinions?