What do you look for in damping factor?

Negative feedback is not normally a problem unless the device is unstable or is over driven (bad design, misuse). Excessive damping factors may be indicative of a design that is not balanced with regard to other factors, such as stability, distortion at clipping etc.

Extremely low damping factors will cause frequency response to vary with speaker impedance.

Audio is represented by a voltage signal. In order to preserve this you should have low ouput impedance compared to the speaker input impedance, which equates to a high damping factor (at least greater than a nominal value of 10 in order to be inaudible when matched with a load that varies significantly with frequency,such as most speakers).

A solid state amp with a damping factor of 25 or lower will sound more tube-like. I have a Onkyo A-9555 Integrated amp and the damping factor is only 25.......sounds very tube-like too!

I'm not sure you can equate a high damping factor with large amounts of global feedback in all instances. Digital amps may be the exception in that they have a high DF of a 1000 but they don't seem to suffer in sound quality, the lack of openess or transparency. I know for sure when a conventional amp is designed with large amounts of global feedback it is usually involves a poorly executed circuit design. So it is my opinion that if your looking for an amp with a high damping factor that results in visegrip control of the low frequency drivers without sacrificing sound quality you may want to go digital.

For example I have used the McCormack DNA-1 Rev B. This amp uses no global feedback, has a low damping factor of 150 and seems to have unlimited power reserves. Because of the low damping factor the bass at times can be a bit mushy & loose depending on the preamp used but definately excels in soundstaging & sound quality, one of my all time favorites.

Having Zu Audio Presence 16 Ohm 101 Db I have been told by Zu DF must be <10.

I just bought the ASR Emitter 1 Ex. With a DF @ 600. Zu says the bass will sound bad. I don't get way having it's own sub amps in each speaker the Presence speaker will not work with the ASR amp.

I hope Zu is wrong and I don't need to sell the speakers.

Audiokinesis, high damping factors don't automatically indicate high global feedback.
So high damping factor does not necessarily imply sonic trade offs in other areas.
As you've posted in earlier threads, it's easier to make a speaker with a low impedance to have an ultra smooth impedance curve, so there are no impedance curve bumps to modulate the frequency response. Typically speakers with such a low impedance are driven with amplifiers with high damping factors.
Are you suggesting that your bumping the impedance in the bass region to correct for a fundamental flaw? Using a port will of course compromise phase and time accuracy.
Th impedance curve of the Maggie's will work well with high damping factor amps well.
I would suggest choosing a speaker that fits into ones budget, room accommodations, listening tastes, then choose the appropriate type(s) of amplification, and then make ones final amplifier choice.