I have been dealing with this issue for a while! I have had the rasp since I put test pipes on. I just recently put on the Stillen CBE and the rasp is crazy loud now. I did some throttle manipulation a couple months back to combat the back pressure, it worked great but my gas mileage went to **** and I was getting some weird side effects with throttle response and a lot of backfire.
I am really beginning to believe it is the resonators; not by their design but by the exhaust design. Your stock cats are doing a really good job at creating a "buffer" to a stable atmosphere. Removing them seems to be creating a high flow which in general is good. From the point at which the exhaust splits all the way back to the exhaust tips seems to be the issue...in this case pressure differences. Just like when you crack a window on the freeway and everyones ears explode, it seems the pipes are fighting each other. I have spent hours trying to figure this out, I am getting a clear sound coming straight from the resonators when I use a stethoscope...no where else!
I am really starting to believe '10Anamoly 's theory with using a pipe to bridge atmospheres...check it out if you already havent read it.
Fancy Tech: How to Get Rid of Exhaust Drone, works great!
Bottom line, this issue is driving me MAD!