I believe it is quite common for the transformer to hum very slightly, with your ear right on it. I would not accept any hum if it can be heard from several feet away in a quiet room. I encourage you to speak personally with the customer reps from Outlaw about this. They routinely bend over backwards for their customers. And they would know the incidence of any abnormal sounds.

Ran the monos all weekend. No buzz or hum. Even with ear to unit.

7 M200's, I don't hear any hum.

You should call Outlaw's customer service. They are extremely helpful. If there is something wrong with the amps (which seems unlikely as all three are exhibiting this issue), they will replace them. If the problem is coming from something else in your system, they will work with you to isolate and fix it.

I was never able to solve the transformer hum problem in the amps.

Outlaw was unable to help?

Was this more than one M200?

Your power line may be causing the buzzing. Some days my balanced power supply hums like crazy, other days - dead silent. It seems to be related to the "condition" of the power. IMO, The important thing is that you cannot hear it from your listening position. Not to say the transformers are perfect - if it is objectionably loud, I would pursue getting replacements from Outlaw.

I have 2 and never noticed a hum.