No, it won't damage your headphones, but that amp is far too powerful for your very sensitive headphones, the extra power is not needed at all.
I'm not sure about the Valhalla's output impedance but I'm pretty sure is not the lowest, that's why these kind of amplifiers are noticeably better with high impedance headphones. For your headphones you should look into amps with output impedance under 2 Ohm (Most of them solid state amps)
Something like O2, should be better in order to preserve the headphone sound signature. In fact you can get great sound quality using a Fiio E10K (DAC/Amp), it has a LOT of power to drive Momentums, and as far as I've read also a very clear DAC.
If you want some technical insight, here you can read about the effects of output impedance on headphones:
http://www.innerfidelity.com/content/meridian-explorer-case-study-effects-output-impedance
Best Luck!