The whole power amp damping factor goes out the window after about 10' of speaker cable. It doesn't really matter how good of an amp you have, damping factor will be minimal after even a few feet of standard gauge speaker cable. It is only really important on subs and the goal would at that point to connect the output of the amp straight to the speaker terminals!!!!!! I.E. don't bother with it as determining factor unless your life depends on it......... Although when it does count, more is better, but plan on using VERY short cable runs to utilize it.
As for speaker cable runs vs. XLR runs, the better bet is always XLR. It's cheaper per foot, has better signal retention over distance and is smaller than a cluster of speaker cables. Speaker cable has this nasty thing called resistance. It is literally sucking power/wattage from the speaker every foot you run!!!!! How much you ask? Well it depends on a few factors. The gauge of the cable, solid core, or stranded core, speaker impedance, amount of power your pushing through it and length of cable all factor how much power will actually reach the speaker. In essence anything over about 25' will start to exhibit a marked difference with small gauge wire. With more robust gauge wire you can run longer distances with less loss. How much loss is acceptable? Depends on how picky you are? To me anything more than a 1db loss is not good. It doesn't take much to get that 1db loss! A 1000 watt amp pushing through standard 16 gauge wire at a 100' length with a 4 ohm speaker load will loose a little over 300 watts of power to the speaker! Changing only the speaker impedance to a 2 ohm load, you will lose almost 500 watts!!!! In both cases you would have over a 1db loss with almost a 3db loss pushing the 2 ohm load! So you can see how running long speaker cable runs can significantly reduce the actual power seen at the speaker. I don't like to use anything longer than a 50' speaker cable if I can help it and prefer to use 12 gauge wire. In the same scenario as above you would only lose about 140 watts with a 12 gauge wire at 100' with a 4 ohm load. A 2 ohm load would lose about 250 watts and there is less than a 1db loss with either ohm rating! The unfortunate thing is that the higher the amp wattage the more power you lose. Using the first example you would lose almost 400 watts with a 2000 watt amp!
This is the predominant reason I am a fan of self powered speakers. The amp is literally " actually " built into the speaker and the speaker cables runs are probably right around a foot! Most are using 16 gauge wire and that length is short enough that the damping factor of an amp is usable and relevant. The down side is sending power and a signal cable to the speaker. If you were to plug in the scenario from above with only a 1' speaker cable you would lose just over 1 watt of power! Now from a wattage standpoint it's not that big of a deal. But it's nice to know that the amp is putting very close to it's rated power into the speaker and not into the speaker cable.