Interesting. So you've had a discussions with them where they said they tuned the headphone output according to the HD800?
One possible advantage of output paralleled resistors is that the amp will see a fairly consistent load regardless of the headphone impedance curve. A downside would be reduced power (possibly a negligible concern), and I'm not sure how damping factor is affected here.
Typically a higher output impedance can give a looser or warmer sounding bass. Not a hard rule, but it depends how the amp interacts. I also have a Bottlehead Crack which has a 120ohm output impedance and I think it sounds fantastic with my Sextett and Sennheisers.