I have the same problem with the loudness of the case. After playing around with a few other cases, I've concluded that the noise issue has more to do with the construction of the case, and less to do with the fan. The PSU's fan is nearly silent if you take it out and put it on a level desk or something. The only noise I hear is the noise of the airflow. If you install and remove the TigerPro PSU, you will realize that the area of the case where the PSU goes into, has a support extension, but the extension don't level with the PSU--only one end actually touches the PSU. I have an upgraded stock fan from AMD, which has copper contact, the fan again is very quiet.
The noise one notices with the Comet case is the rattle sound from the "not fully supported" PSU, and acoustic amplification of the case construction. I've installed the same MB, same stock AMD fan, and same TigerPro PSU in a different case, and the result was near silent. The Evergreen? 80mm fan that came with the case also produce significant noise and rattle and vibration.
PS: Has anyone notice how crappy the power button is? It's SO big, that it gets stuck sometimes, but when you try to loosen it up, you might actually trigger a brief contact, causes the MB to see it as a shutdown...