Generating 5V from 37V DC with no EMI
Look at it this way - you've found a weakness in your amplifier so, if you solve that you'll be in a much better position to cope with all sorts of EMI issues that can occur from time to time in pretty much all designs. 

EMI is always produced by a switching regulator so your options are: -


Choose a linear regulator (and a big heat-sink with significant power loss) or, 
Solve the basic problem with your amplifier or,
Pick fans that run on a higher voltage