I've recently given my rig another thorough clean inside with a gentle brushing, especially to all the fan blades, followed by a careful compressed air blast to clear out the residue. Now it is back to running as quiet as when new. TC.
I'd looked at QuietPC as I always try to buy from them first. The problem with their modular silent PSUs is that they're only silent at idle. When you put them under load the fans can spin up to 30db or more. If you work through the specs you can see the claimed noise ranges vary quite a bit. The fanless ones aren't modular.
The Xilence range 'looks' good but the fan on mine has got noisy and 135mm quiet fans are nowhere to be found. I did try to source one from QuietPC. The Xilence claimed cfm and db ratings are a bit 'iffy' too.