Math.Net and alglib returning different FFT outputs by default
I worked it out by myself, after reading a bunch of posts mentioning different methods of FFT output scaling. I still find this aspect of FFT processing heavily unsdocumented everywhere. I have not yet found any reliable source that explains what is the use of these scalings, which fields of sciences or what processing methods use them. 

I have yet found out three different kinds of scalings, regarding the raw FFT output (complexes' magnitudes). This means multiplying them by:
1. 1/numSamples
2. 2/numSamples
3. 1/sqrt(numSamples)
4. (no scaling)

MathNet.IntegralTransforms.Fourier.Forward function (and according to various posts on the net, also possibly Matlab and Maple) by default, uses third one. Which results in the better distinguishable graphical output when using logarithmic colouring, in my opinion. 

I would still be grateful if you know something and share your ideas, or if you can reference a good paper explaining on these.