I now face a new and challenging problem with my two model SE84UFO2.1 amps.
Together with my 94dB or >100dB loudspeakers there is a loud or louder and audible hum coming from the loudspeakers.
The hum is less audible coming from the 94dB speakers.
Using the >100dB open baffle speakers is no fun … when playing soft/classical music the hum is an additional instrument that plays its own song.
So how to solve his?
Regards Bart

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Hum is a trigger for me, so I feel your pain. Unfortunately there is no single cause of hum and no single solution.

Donnie's video about ground loops is one common cause, but I didn't watch it so apologies if the following was covered in the video.

It would also help if you can isolate the hum. If you disconnect all the inputs from the amps does the hum change at all? If the hum is the same with no inputs connected then you do not have a ground loop. If it goes away you might and presumably the video will give you ideas on how to fix it. Or, the noise could be coming from one of your sources.

What about tubes? Have you tried installing new/different tubes? Note that if you don't have at least one spare set of tubes you ought get some, because tubes can die or otherwise go bad at any time.

A common source of hum is bad/dirty power. Is it possible to try at least one of the amps in a different house? Or at least on a different circuit in your house. A solution for dirty power, and just an upgrade in general is a so called balanced isolation transformer. Even though hum is not a problem for me getting one from ebay aka China improved the sound of my entire system.

How long are your speaker cables? Speaker cables, esp. if they are long, can be an antenna that picks up electromagnetic noise from your house/neighborhood. One of the big reasons I prefer monoblocs is because I can put the amps close to the speakers and use very short speaker cables. That just sounds better, regardless of noise issues.

Beyond that it is always possible there is a problem with your amps, and you would need to get Steve involved. For me, I have a pair of UFO25s, configured as balanced monos and they are very nearly dead silent with my 95ish dB speakers, as was my pair of UFO amps before them.

Keep us posted.