Great Work fellows!
The reason for the Bessel filter is that it is the only filter that it has flat time delay .With all the others, Butterworth , LR, etc. the time delay changes with freq. With a Bessel filter , the one, and most important thing is to have a LF. load Z that terminates the filter with a flat freq resp. Hence the Zobel.
Please check the time delay with a scope and the test tone from my web site

www.donpatten.com

Don