The Macho is the untouchable king of low airflow.
But is it quiet ?
The Macho comes with a fan that is reputed to be quiet. However, I don't think you'll even need that one. The heatsink is build in such a way that it rests only a few mm in front of the rear exhaust fan and the airflow of that should be enough for your non-overclocking endeavours.
On another note any ideas on how the stock Define XL R2 fans perform in terms of quietness ? Looks as though only one Front fan is supplied, along with rear and bottom (pulling I assume). Would swapping the bottom fan to the front position be a wise move so I can get more air flow. Or perhaps a 4th fan ?
Whoa nelly, the Define XL is a very big case. Sure you want/need one as big? Your setup does not need it.
If you take it: The traditional way to cool the CPU would be to plug it into the top PCI-E slot and have it cooled by the upper front fan. Remove the drive cage for optimal air flow. Should be sufficient. A bottom fan is NOT needed in your setup. Place it in the lower front and set as slow as possible to cool your WD Greens. They need very little airflow as it is.
You could experiment with using the bottom fan to cool the GPU. For that, I would plug the FirePro into a lower PCI-E slot (though beware of their speed, could be that the lower ones are only PCI-E x4), the closer the better.
In this scenario, the upper front fan would not be needed.
I would imagine the default fans that ship with the case are quiet if you control their speeds. Asus usually has some of the better fan controls available. At speeds over 600 rpm, no (120mm+) fan is silent. At over 800 rpm, none is quiet. To my ears at least.