raoultrifan
Headphoneus Supremus
I don't think output impedance is directly related to "hiss". Of course, 9-ohms headphones are usually very sensitive and some hiss might be heard, but this is mostly related to how the amp pairs with the headphones used and also internal noise provided by voltage-amplification stage and gain used and output buffer's noise too. I would suggest you to try before buy, if possible, because if we're having a look over TI's datasheet for TPA6120A (http://www.ti.com/lit/ds/symlink/tpa6120a2.pdf) then the minimum recommended headphones impedance would be 16 ohms, so theoretically neither Mojo, Air, UA-M10 etc. should not perform admirably on 9-ohms headphones (which is not quite right, of course, because TI's datasheet presents a schematic having a 10-ohms output impedance and a good designer could use low-impedance output inductors to lower any possible oscillations). So, please give it a try if possible and post your findings here.
A 1/8 rule between amp's impedance and headphone's impedance would be wise, of course, but mostly because the voltage that drops on internal's output resistors/inductors will be higher for amps having a higher output impedance. Most likely frequency linearity might suffer if output impedance of the amplifier is very close to headphones impedance (http://www.meier-audio.homepage.t-online.de/tipstricks.htm and http://www.innerfidelity.com/content/musings-headphone-amplifier-output-impedance#GipPCiW1utzugX1r.97 explain this very precisely!).
P.S.: Mojo and iDSD micro/nano have <1 ohm output impedance, so could have a slight advantage over Air when speaking of 8-ohms heapdhones, but I still think you should give it a try to each of the above DACs.
A 1/8 rule between amp's impedance and headphone's impedance would be wise, of course, but mostly because the voltage that drops on internal's output resistors/inductors will be higher for amps having a higher output impedance. Most likely frequency linearity might suffer if output impedance of the amplifier is very close to headphones impedance (http://www.meier-audio.homepage.t-online.de/tipstricks.htm and http://www.innerfidelity.com/content/musings-headphone-amplifier-output-impedance#GipPCiW1utzugX1r.97 explain this very precisely!).
P.S.: Mojo and iDSD micro/nano have <1 ohm output impedance, so could have a slight advantage over Air when speaking of 8-ohms heapdhones, but I still think you should give it a try to each of the above DACs.