I was recently setting up the filters on my Elecraft K2, using spectrogram software, and I got to pondering whether it was best to center the received signal in the pass band using a log or a linear frequency scale. This lead to a chain of thought that has caused me to start operating with a much lower received note than I had in the past.
Human hearing has logarithmic response, in both amplitude, and frequency.
Though other factors play a part, about 100 Hz is the minimum practical bandwidth for CW reception. Note that this is stated in Hz, not octaves. For a given bandwidth filter, an equal amount of QRM and QRN will come through along with the desired signal. If the pass band is centered at a lower frequency, then all the interfering signals will seem much more spread out, and the desired signal will stand out more. Also, when using wider filters, if the received signal is centered on a linear scale, then nearby stations are as likely to be above, as below the desired signal.