The BiquadFilter node is changing the formulas for the lowpass and highpass filters.  The basic affect is that the Q value of the filter produces different filter responses now.  From some experiments, we know that Q values above 5 or so have only small audio changes.  Smaller values, however, have easily noticeable changes in audio.

To understand how much this change might affect existing applications, it would be useful to have a histogram of the Q values in use. 

This is a bit complicated because the AudioParam object doesn't know what it is being used for and because the Q AudioParam is used for all 7 filter types.  We only want a histogram for the lowpass (and highpass) filters.