If you plug the HP out to a headhphone amp the signal that it would get is the one measured in your chart. The final output would depend on the headphone amp specs.
To cut a long story short: the signal measured in your chart looks clean enough to be amplified once again (by the headphone amp)..BUT (a big but) it would always be better to feed the amp via a clean line out (like the iPod Classic one for instance). And take into account that amping a phone is problematic because of RF interference.








