Speakers play a big part here.
PMC are 8ohm, the OPs Sonus Faber are 4ohms nominal impedance.
So despite being similar sensitivities, one will present a more difficult load to an amplifier than the other. The SF dip to 2.8ohm.
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Itâs difficult to overheat a 250
but then sonus Faber is difficult to drive also
all said and done itâs a domestic amplifier not a PA rig
I have not read this thread in all its length but I have used a lot of vintage amplifiers in my time. In Denmark where I live it they changes the network voltage from 220 V to 240 V some of my older amps did like this rise in voltage so I bought a variable high capacity transformer and turned down the voltage to 220 V. After that I have no issues with overheating amps or amps turning on protection circuits. My point is that you might want to check your voltage output and if to high correct it with a transformer. It might be a very cheap trick.
Interesting - did not know that.
How is my 606? 4ohm or 8ohm?
Some are 4Ω
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Reviewed by Hifi Exclusive - the impedance chart has a min of 7.5 ohms (180 Hz) and a peak of 23 ohms (1,900 Hz).
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It should, however, happily provide up to its rated power/distortion levels for hours on end without complaint, provided it is installed/ventilated correctly and assuming the manual doesnât expressly say otherwise.
FWIW a few decades ago I had a Radford 50W amp for my hifi, and used the same amp for the rock disco I ran, on a few occasions winding right up to absolute maximum, with no signs of distress. But being horns the speakers were both an easy load and very high sensitivity (105dB/W)
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What does it mean - itâs less demanding? But with 500dr it should be no problem. my 500 gets warm - but only in summer 
You use nc250 - this should also be perfect with 606
My reply was to @Hagel whose 25 24i are 8ohm.
piece of cake to drive!
SF are well known for loving power delivery.
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I can only quote what it says. Iâm no electrical expert. But at least it doesnât dip low, which is often mentioned as a challenge for amplifiers.
My NC 250 drives my 606s fine. But then Iâve never been past 42 volume (range set to 100).
Indeed, it would seem so, although I have no personal experience of SF speakers.
Btw - itâs a very positive review of the 606s by Hifi Exclusive.
Currently, I have the NC250 connected to my home made speakers which are 6 ohm Scanspeak Revelators. They must be a real good load for the 250, probably similar to the PMCâs.
Do not find the review - can you help searching
Check the Kudos site for the 606. Scroll down to reviews. Stereo Magazine.
I maybe misled you with âHifi Exclusiveâ which appears at the top of the pdf.
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Itâs a German magazine - read this long ago 
We all realize donât we when a speaker says say 8 ohm⊠that is simply a pass band average, and the actual impedance will usually be varied, sometimes quite significantly based on enclosure design, crossover design and the number of drivers as well no doubt other parameters.
The actual impedance the amp sees is the cross over impedance rather than specifically the driver impedance.. some may be close, others could be quite divergent based on speaker design.
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Probably not, although we do all wish we had a Joe 90 plug for you!
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Very true, @Simon-in-Suffolk . And donât even get started on EPDRâŠ
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I have SF Olympica Nova 3âs myself. My listening sessions, commercial reviews, have convinced me that the NC 250 is âjust barelyâ enough to drive the SF3âs.
They need 350âs to shine. And thats already at normal loudness levels. If you really go load the NC 250 really has to work.. With a hardware failure like a not working ventilator you risk going into unwelcome situations faster.
Hope you can have your tweeter fixed, the SF Nova 3 is a great speaker.