I'm assuming they are already able to be underpowered by reducing the supply voltage since they are variable speed.... so that's not going to help...

You may not like this either then.... most of the noise is created at the outter most edge of the blades. The closer tolerance between the blades and the interior of the fan body, the more efficient but more noise is generated... if the blade is out of balance it will create a ruckuss as well. If you were to shorten up the blades a mm or so it may quiet them down considerably. I've used this method on computer CPU and case fans; insert a piece of sandpaper in there and turn the fan on... it'll shorten up the blades a bit and blance them at the same time... but you will sacrifice some performance of the fan for quieter operation. The only real problem is you obviously can't put back what you remove if you don't like the results.