Isolation techniques are all well and good, but how about using some really good fans to start with (i.e. not RS crap)? I recommend Pabst, the original "whisper fan" they're so quiet and vibration free you won't even know they're there. It sounds like you already put in a lot of time and effort, so don't "cheap out" now!
http://www.ebmpapst.us/
There are many suppliers. Type:
pabst fans
into Google search.
.
http://www.ebmpapst.us/
There are many suppliers. Type:
pabst fans
into Google search.
.