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[Confused]

OK - I will now repeat now my subjective experience of listening to a D170 versus a D240 Expert with the KEF Blade - The D250 was substantially better.  In fact, the difference was so stark that I believe that for some reason the D170 was not driving the Blades properly, the main issue being a complete collapse with the bass definition.  Do you believe me?  I am just a bloke on a forum, I could be making it up.  Maybe I just thought the D250 was better because it was more expensive?  So you could read this and shout "rubbish", they sound the same, you week fool, you have been struck by the 3P's.  Maybe.  Of course, I have the advantage that I was there, so I am clear in my own mind what I experienced, but for your lot, how do you know?  I could say in my defense that I wanted to try the D170 because it was cheaper, and I thought that the drop in 70watts would make no difference, my expectation bias was that they would sound the same.  But you could come back and say that the 3P's had struck me subconsciously.  The truth is, nobody can prove this, and I mean 100% prove this either way.

If SAM was on, then the difference between the two amps was much more than 70W. 

With its 4 boomers, the blade is certainly deeping quite low and therefore peak power is what matters. 
Ask yourself why a 250 can be bridged into 1000W with peak on 2W over 4000W while a 220 will only bridge to 440 and peak on 2W of 1600W. 

So although the constant power between the two products is within 0.5 dB, their peak power is 4dB apart. 

That’s probably what you have heard. 

Jean-Marie

As for SAM, there was a clue in the words, this was a comparison between the 170 and 240, hence before the glory days of SAM.  That said, I am actually quite pleased that you picked up on this point.  The thing is, I can do the calculations, the Blade has a sensitivity of 91dB/1w, so with a 170w amp, each speaker should run to over 109dB(a), or 112dB(a) for the pair, which should be enough.  Enough?  Yes, enough to cause hearing damage very rapidly. 

So why did the 170 appear to struggle with the Blades, the impedance curve does not dip too low, 3.2 ohms minimum, which is fine for a Devialet.  You mention the 4 boomers (which I presume are bass drivers), and yes, I wondered about this too.  Four per side is a lot, but why does this matter if the speaker efficiency is reasonably high.  I am sure that we could speculate on this point, the effect of multiple voice coils, the fact that the Blade uses very large voice coils (about the size normally found on a 15" driver), perhaps something to do with the behavior of the crossover.  The thing is, I do not know.  So this is exactly the kind of thing I was talking about when I said audio is complex.  I am sure if we could get one of the KEF designers together with one of the Devialet designers, talking freely, then we might get some idea of precisely what is happening.  The rest of us, we make an assumption based on speaker efficiency and how logarithmic sound pressure levels work (which is basic engineering) and we get the wrong answer.  Not because the basic engineering assumptions are wrong, but because there are other factors involved.  These factors are based in science and engineering too, of course.  It is just that the interaction between speakers and an amplifier is more complex than just watts and efficiency, there are dynamic factors going on that are probably very difficult to predict or model accurately.

So, without having a team of top hifi design engineers to help us, the best we can do is use our ears and have a listen, and see if it works.  Of course, we need to watch out for expectation bias, and the reason I found the D170 did not work with the Blades could all be in my imagination.  Myself, I am 100% sure it was not imagined, but I could never prove it to someone who reckons that I had.