Probably not an exact science but is it possible to guage how hard an amp is working from the casing i.e. how hot it gets ? If I play a 60 minute heavy rock CD at nine o’ clock on the 250DR in to NEAT Petite 30’s at the end the 250 seems rather warm in fact very warm !
There are a few factors that will have an effect on how warm/hot your NAP250DR will get; the speaker cable, the speakers, how hard you drive said speakers, and the ambient temperature and how well ventilated is the casework (this is the heatsink).
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I’m impressed you can even make it to 9-o’clock. You obviously have no neighbours
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The Neats are 4-ohm and cited 87db efficiency which, in relative terms, is not an ‘easy to drive’ set-up. This will mean your 250 with a digital ‘line level’ source playing dynamic rock could be working hard at 9pm = heat.
The primary reason IME is likely to be the 4-ohm loading, which means current is being sucked from the amp. I think Naim amps prefer 8-ohm speakers.
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I think Naim amps prefer 8-ohm speakers.
?
Which technical reference can you provide for that?
The key word is “think”, It’s an opinion.
Naim’s own speakers were between 4 and 8Ω but with no nasty phase angles.
Stereophile tested the NBLs with this result.
Which is very amp friendly but mostly 4-6Ω.
The Neat iota α looks like this
and there’s nothing too disastrous there either but not as, er, neat as the Naim’s.
There isn’t a plot for the petite unless another mag or Neat have published one.
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You might extend this, to say - most amplifiers will ‘prefer’ 8 ohm speakers…
It’s simple Electrical Engineering - a 4 ohm speaker (of the same efficiency) will make any amplifier work harder than an equivalent 8 ohm speaker.
Also, a less efficient speaker will make the amplifier work harder, than a more efficient speaker.
Less efficient often means a speaker with a complex arrangement - of drivers and/or crossover. BBC derived designs are some often found in this category. Isobarik’s are be another.
So, a high(er) efficiency 8 ohm speaker is the way to go…!!
Common Sense - or it should be, around here… 
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The key is how amps deal with impedance swings/drops.
naim put great stress on the fact that 250/300/500 can drive 2 ohm loads for extended periods with no ill effects.
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No argument with this. Believe Naim used to (maybe still do…?) test their amps into a 2 ohm load…?
If you must have a complex and/or low efficiency speaker, then its probable that a Naim am,p will make a better job of it, than most. BUT - it will likely get hot(ter).
(Hence, why 135’s had fan’s. Maybe 300’s do, too…? Unsure about 500’s…?)
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250DR into 4 ohm Sonus Faber after extended listening at loud levels is definitely warm (but not hot).
The fact 300DR has a fan is definitely another reason I can propose to SWMBO to justify an upgrade 
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Can you go too far though? I’m not sure how a certain member got away with 105dB/W Grandinotes on a 552/500, let alone the Statements he replaced them with. Unless Grandinote’s 105 is on the optimistic side.
Towards high efficiency…? Guess so.
Grandinotes are an unknown to me, sorry.
Dangerous ground… 
I’ve been quite disappointed that I can’t get my 250DR very warm. I’ve tried extended loud rock sessions (10:00 plus on the dial) and it hardly sweets. My Nova, on the other hand, if I drive it hard, it gets properly hot.
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Perhaps not loud enough rock 
If you’d like a list of bands my wife describes as “not music”, “wall of noise” or “too screamy” to try and heat up your 250DR let me know
Maybe we need a new thread on the Music section - “Albums to get your amplifier really hot”!
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Well at-least I don’t need a 300DR, the fan would never come on 
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I run 272, 250dr and Kef r3’s. The power amp barley warms up but the 272 gets pretty warm. This is a new one on me also.
Most likely its Digital circuits. Different question…
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Thanks, it doesn’t bother me, just seemed odd to me.