Audio Alchemy OM 150 hum. What is the cause?

If it is a low frequency hum, it is most probably a ground loop somewhere, unless the problem is within the unit itself. I suppose you have to get a cheater plug and experiment with the powercords, taking care not getting electrocuted. See to it, that all your units are plugged into the same wall receptacle and experiment with a grounding wire between your OM 150 and your preamp. If all that doesn't change anything, try another pair of interconnects. (Sounds crazy, but for me that helped once)

I have tried the cheater on both the amp and preamp. I haven't changed ic's yet. The hum is much higher than a 60 cycle hum. More into the midrange.

Larry, sometimes you have higher harmonics of those 60 cycles, but the cause may still be a ground loop. Do you get your power to all your gear from the same outlet in the wall? If not, try that. I would also experiment with a piece of grounding wire from the chassis of the amp to the chassis of the pre. Make sure to get good contact. If all that does not help and changing ic's does not get you anywhere, possibly it is the amp after all. By the way, is the noise more of a hiss or buzz or is it quite clearly hum?

I would call it a buzz

Well I don't know, it could very well be the amp after all.
Have you tried it with a grounding wire, as I had suggested?