From what I know, in speaker systems the separation between the power amp and the pre-amp is strongly advocated, as it further separates the initial signal and the final amplification, forming a cascade of amplification, giving a cleaner sound signal with lower noise and interference and sometimes it's used to increase voltage gain. (Please correct me if I'm wrong)
However, in headphone systems, people usually feed the signal directly from the source to the final amplification. I don't see many pre-amps accompanying a Lyr or a WA5, for example... Why is that? Is there technical reasons? I would appreciate any answers, thank you!