Can filter "depth" be adjusted by mixing dry and wet signals?
adding the input to the output of a scaled 2nd-order bandpass IIR will get you the classic peak/cut EQ curve.

adding the input to the output of a scaled 1st-order LPF or HPF will get you a 1st-order low-shelf or high-shelf EQ.

adding the input to the output of a scaled 2nd-order LPF or HPF will get you a particular form of a 2nd-order low or high shelving EQ, but there are many others.  doing it this way will often leave an unintended null or lump in the frequency response.  if you want a symmetric (in log frequency) and perfectly monotonic gain vs. frequency curve, i might suggest referring to the Audio EQ Cookbook.