Audio Research Reference 150 - buzzing input transformer

Hello everyone, 

I have bought a second hand ARC REF 150 power amplifier (non SE, KT120 tubes) and there was a buzzing noise coming out of the input transformer after few hours. I have detected the noise appears as the amp fully warms up - after about an hour. It goes pretty loud within another 30-45 minutes.

I have taken the amplifier to a friend, a reputable tube amp engineer. He told me the input transformer must be replaced and also that in his opinion it was poorly designed.

I live in Europe so I didn't have the chance of sending the amp to the factory, nevertheless, a new transformer is on its way and I'll get it fixed by an authorized dealer and service. 

I was also told that the problem may happen again and that I should get rid of the amp. 

My question is whether any of you had similiar problems and what the cause can be. I really love the sound of this amp and of course I'd like to keep it. I'd also like to do as much as possible to keep the amplifier safe. 

I have found a very similiar discussion here on Audiogon, I have messaged the OP and he replied he had his transformer replaced afterall - 

Is there anything that can cause the transformer to overheat and melt inside? The noise was caused by a vibration of the plates inside the transformer. 

My speakers are Tannoys D700, they go down to 3 ohms and I can imagine they are current hungry even tho their sensitivity is 92dB. They worked well with a Classe CA-200 but the ARC sounds a lot better of course. I'm planning of upgrading the speakers in about a year but for now, I'm still very happy with these, it's always a pleasant surprise when I upgrade any piece of gear. 

I have asked the ARC company whether the speakers won't be a problem for this kind of amp. They ensured me I'd be fine and that they test their gear with much more difficult loads. 

The amp never got too hot when I placed my hand on the top of the chassis, the tubes were measured and they're fine, about 1100 hours on them. The amp wasn't in use for a few years before I bought it. 

Thank you for any suggestions.