My system is Core 2 Due E6600 in Asus P5B-E Plus with 2GB RAM, X1950 pro and 500GB HDD. The case is a Cooler Master ATCS 201C modified to take 2 rear 80mm case fans, the front fans have been removed and the front grill replaced with free flowing mesh. The “blowholeâ€

Nice job. Though I must say you must have some kind of super hearing because I have two NeoHE 430s and both are silent.

Maybe you had a bad fan?

Maybe you just haven't heard it yet. It's a blessing.
(I don't mean that it's loud, but it can be heard, maybe bad fan batch...)

I also believe that the NeoHE can be improved upon, particular that Adda fan. It's good, but not the best, and it's ball-bearing.

And required air flow - in the review it does 20dbA up to 21dbA @ 150W, that's like a low power CPU with mid-range gfx (or more than that), @ full load.

Oh well, anyway, I just chunked a Panaflo "L" in mine and forgot about it. Warranty destroyed, but hey it's a Seasonic and it's not my main com.
Gonna get a Yate Loon into my S12 right after warranty is gone. That's for my main rig.

Nice job.

Just this morning I was thinking that there are 80mm to 120mm fan adapters, and why couldn't someone remove the 80mm fan in their PS, replacing it with a 120 that sits outside of the chassis?

Because you don't get any additional airflow out of it unless you increase the speed of the fan to overcome the additional backpressure, defeating the point of using a bigger fan in the first place.

Put another way, you have an 80mm fan and a 120mm fan that both push X CFM in free air (unrestricted). The 80mm will have to spin faster to produce X, due to smaller blade area. Using 120mm fan allows you to run at a lower speed to produce the same CFM...in free air. The only problem is because of the blade spacing and speed of the fan, the airflow has less pressure behind it than the airflow from the 80mm would. But when using an adapter like that, you are trying to squeeze a "120mm-sized" airflow into an "80mm-sized hole". The compression of air pushes back on the fan (called backpressure) preventing more air from flowing through, in addition to creating more audible turbulence noise (and possible bearing noise by physically pushing the yoke into the bearing shaft). To overcome the backpressure you either need to add more blades (obviously you couldn't do this with the same fan) or speed the fan up.

So again, those adapters are worthless save in an extremely confined space with no other alternative. You're better off finding an appropriately sized fan and working with that.