Anything over 50 is gravy. I'd like to take this time to dispel another myth, that super high damping factors make an audible improvement. A damping factor of 50 means that the amplifier's actual output impedance divided into the load impedance is 50. In this example (he was testing a Diamond Audio D7 amplifier for Car Audio Magazine) the amplifier's internal impedance is .08 ohms. A typical four ohm speaker typically has 3.2 ohms of internal DC resistance. So, the actual damping factor could not be better than 4/3.2 = 1.25! That is why emphasis on ultra high damping means nothing sonically.