What is an acceptable level of transformer hum on a Krell S300i?

Greetings from a newbie. Just a quick intro:
  
Not rich, but I've been into hi-fi since college (Dual CS506 era). 20 years ago, I had progressed to Michell Gyrodec/Rega, Roksan Caspian CD/bi-amp, Sonus Faber Concertos.

The speakers never sounded as good as when the dealer had demo'd them hooked up to a Krell KAV 400i. But this was not a pressing issue as everything went into storage while I went travelling/working overseas.

Fast forward to present day. My brother has just got into vinyl (having discovered a killer high end McIntosh set up while staying at an Air B'n'B), so I unboxed my kit and set it up to show him what he should be listening to.

We both felt the sound was disappointing and my thoughts returned to that Krell demonstation 20 years earlier. My focus then wandered to eBay and I found myself winning the bidding on a S300i amp.

It is in excellent condition but it hums, even when in standby and without speakers or sources connected. I have tried different locations around the house, different houses, cheap mains cleaners and 3 into 2 mains plugs to eliminate the earth. Nothing makes any difference. When I press the mains switch on the back, the amp starts with a 'cough' which settles down to a constant hum after 2 seconds. I think it is higher than 50/60hz. Possibly closer to 200 hz. It does not vary at all after that. I convinced that this is the transformer because it also occurs in standby without any connections.

If the amp is on the floor and I stand next to it, I can hear the hum.   At 15 feet away, I can't really hear it, but with it in the rack and in my seating position, my ear is only 5-6 ft away and it is clearly audible when the room is quiet. It is annoying when reading in silence to the point that I need to disconnect the power.

My question to you... is this an acceptable level of hum? I would have expected that unless I put my ear next to the casing, I would not be able to hear anything. I have hunted the forums and web but can't find an answer for this specific issue. I use Transparent cables but these aren't relevant because it hums without any cables connected.

Your comments/thoughts will be greatly appreciated. I have already raised this issue with the seller who claims that it did not hum when he sold it. If the hum was really loud, there would obviously be a problem with amp. It is at a level that I can't be sure about without anything to compare it to.

Kind regards.