Hey there! I know this is a very old post, but I’ve recently bought a EHX Mic Pre and I’ve found myself with the same “problem” you describe. It’s not a problem per se. It happens that both outputs of this pedal are balanced. The hum noise appears when you you use the balanced 1/4” output and plug into an unbalanced 1/4” input. To eliminate that hum, you have to use a balanced cable (stereo jacks on both ends) and go into a balanced input.

Now, I want to process my voice using a lot of effects pedals (all of them use unbalanced inputs/outputs). The microphone goes directly to the EHX Mic Pre, and I cannot go from there directly to another effect pedal (because of the hum noise). To achieve this without noise, I have to take the balanced out from the Mic Pre and go into a compressor (or a mixer with balanced inputs). In my case, I go into a DBX compressor, that has an unbalanced output. So I use that unbalanced output to go to the effects pedal. In this way, I don’t get any hum noise.

I’ve tried making my own cable TRS –> TS but it didn’t help, the hum noise is still there.
So far I’ve found that the only way to eliminate the hum is go from the Mic Pre to a balanced input on a mixer or whatever device that offers an unbalanced out.

Hope this helps.
Let me know if you can think of another way to go out from the Mic Pre directly to an unbalanced input without that ugly hum noise :/