what make is the after market one.
just out of curiosity do you have stainless headers, downpipe, up pipe, cats ?, middle boxes etc, as if you want to go louder these are the things to change without compromising the overall sound.
For example my bugeye had 2.5inch back, no cats and one middle resonator (silencer) into a pretty small back box (toyosport) and a 2.5 inch uppipe, this gave a very loud but satisfying Scooby burble at idle, then I replaced the headers for unequal length stainless and it made it sound a lot louder but still sounded great.
Now my Classic is the same but 3 inch back into a Jasma backbox which is huuuuuge the overall sound isn't as loud but the tone is a lot deeper.
I guess what im trying to get at is a full system may be more beneficial in the long run if you don't already have one. As always hope this helps.