Amplitude in frequency domain of the same signal measured at different bandwidth (f_max - f_min) and different FFT lines
Are you using the correct FFT form for spectral analysis?  You need to divide by N when mapping time-&gt;frequency domain with an N-point FFT for a power spectrum. If you want power spectral density you have to factor in the bin bandwidth and window effective noise bandwidths.
The amplitude of a spectral line represents the power of a single signal frequency, power spectral density reflects power per unit bandwidth, which is different.