Got a 2006 sti with the standard sti backbox. I'm looking for something I can put on that has just a bit more noise so I can hear the burble but isn't ridiculously loud, Just a little louder ;-). I don't want to replace the entire system or the cat.

 

There are plenty of second hand ones like Prodrive, Scorpion and Hayward & Scott which are only slightly louder than stock. I've got a Scorpion one and it's almost the same as the stock one on tick over but sounds good on acceleration.

 

Milltek? Have had them on my last two Imprezas and they are excellent and exactly as you describe - louder/burblier (think I just made up a word?) but not too silly. Quite pricey new though.

 

If it's got all the cats in and resonated centre still you can take your choice of bbox and it won't be too intrusive