G3VGR K2 Spectrogram Plots      

Here are the actual IF Filter passbands used in the four CW bandwidth positions as shown with Spectrogram. I used a homemade Noise Generator as input to the K2 and fed the audio from the headphones jack to the Soundcard input of a laptop.  This is a simple and extremely effective way to set the BFO frequencies for optimum performance. Elecraft can supply the N-gen Wideband Noise Generator kit for $39, although nothing special is needed.
Tom Hammond, N0SS (SK)'s website contains all the info required to construct an effective noise generator. Also on Tom's site is the Spectrogram program and detailed information on how to use the noise generator and software to align the K2.