using a wheatstone bridge with a .2 ohm resistor in series with the Amp to speaker connection to feed an op amp that corrects for the back EMF gives the best results.Re: Damping Factor - Actual listening tests?
Bennett figured it out and wrote about it:
http://www.bennettprescott.com/downloads/dampingfactor.pdf
Long cable runs with low impedance loads make the lack of damping factor noticeable, increasing power does not fix the problem.