Crossover: IIR or FIR?

[maxijazz]

Im general, in electronics, cut off frequency is the one where there is 3dB fall (or raise) in signal power.
It would mean that 1-st order filter is properly implemented (-3dB fall at 250Hz). The other filters no so well. I have to admit that different types of filters could have different definitions of cut-off

Not -3 dB.

Crossovers are -6 dB/per per order / per octave.  So 1st order is -6 dB per octave....4th order is -24 dB per octave.

Here's what I don't get:

Why would Devialet implement this in such a non-intuitative way?

I am right and you are right. It is misunderstanding. 
I am talking about definition of cut-off frequency. It is at - 3dB. 
You are talking about filters' slopes (roll-off/damping rates). 

Cheers.