I suggest replacing the Delta fan with a low-to-medium-output 80mm fan on a 60->80 adapter. Have a look at what
006agent is selling in For Sale/Trade:
60->80 adapters They look pretty beefy and are $13 plus shipping for one with no fan. For quiet fans, I would try Teamawe.com for the Panaflos, or phamcomputer.com for the Addas, or there is PC Power & Cooling's silencer, pcpowercooling.com or directron.com have the Silencer. I'm prejudiced toward the Panaflo L1A but the Adda has a bit more rated output and still is under 23dB... super quiet. The Sunon low-noise model made too much motor whirr for me, don't like that one's noise level.
Phamcomputer says the fan on the Volcano II doesn't seem to bear out the promise of 36cfm at 31dB, if you have a look at their front page remarks.