My 1st tube amp: transformer buzz audible 15 feet?

What is your amp sitting on? I had a similar problem with a more or less new RM-10. It was my first tube amp as well and I could hear it 10 feet away. I had it sitting directly on a piece of ply on spikes on a rug over concrete. I believe it was resonating with the ply and amplifying the existing hum--and it was annoying as hell. I happened to have some herbies tenderfoots I wasn't using so on a whim I stuck them underneath the amp. Problem solved. I can still hear the same hum if I stick my ears right up to the amp but it's no longer audible from my seating position or even 2 feet out. Nothing magical about the tenderfoots, because I switched them out (tonight as a matter of fact) with some audio technica footers I have and same result: quiet. Try it ASAP. I suppose trying a different platform might work as well.

If it doesn't work then, as most everyone has said (and obviously correctly), return it.

It sounds like a defective transformer. Is this unit new? Who makes this unit? A mild buzz is not uncommon in a unit that's a few years old.

I have a pair of monoblock amps and one of them had a slight buzz that eminated from a transformer. The buzz was related to a quality control issue with the epoxy inside the transformer in a small batch of transformers they had received right around the time my amps were built. If the transformer didn't develop a buzz within a couple of weeks, I was told it never would. The other amp was perfectly silent, by the way.

Based on my experience, I would ask your shop to return the amp to the manufacturer diagnosis and replacement of the transformer if necessary.

I have had my amps for over a year and both remain silent.

Absolutely return the unit.