I guess that I've had a similar experience with the Neuspeed exhaust.

The first time I drove it, it seemed incredibly boomy at low RPM, especially when cold. It still seems louder than stock, but I think that I'm getting used to it.

My wife usually only rides in it on weekends, and although she didn't say anything, when I asked her she thought it was loud, but she's used to her Solara coupe. Last time I drove that, I was amazed at how quiet it was, even with a TRD exhaust!

A friend rode in it this week who's only ridden in it once before and he said that he couldn't tell any difference.

I've got to think that it's mostly acclimatization, you just get used to it after a while.

 

Perceived attenuation? Gee, I love it when you talk dirty!

 

My HKS exhaust was a little noisy for the first week then it calmed down . I think a few good thermal cycles relieves some stresses in the exhaust.

Also the hangers on mine were a little tighter than they were on the stock exhaust.
Once they stretched a little, the exhaust loosened up a bit.

I have no complaints other than the fact it didn't give me 25hp!

 

I hear ya, those are like 25 cent words!

 

I thought so too, but a friend of mine that complained about the low rpm drone a day after I installed my Mugen exhaust couldn't find any reason to complain after riding in the car a few weeks later...