Your best bet is to ask on "Head-Fi.org" many expert "can" aficianodos are there. My own experience is that the headphone output circuit on a CD player, seems any afterthought and not an important feature.
I own a now disgraced but really incredible sounding dedicated OTL headphone amp by SinglePower. I have owned a couple and can tell you as a rule they are better than any other combination source.
There appears to be some preamps however like the Cary SLP-05 which have a dedicated headphone amp in it. I would imagine there are others like that on preamps, but never seen it on a CDP.
I own a now disgraced but really incredible sounding dedicated OTL headphone amp by SinglePower. I have owned a couple and can tell you as a rule they are better than any other combination source.
There appears to be some preamps however like the Cary SLP-05 which have a dedicated headphone amp in it. I would imagine there are others like that on preamps, but never seen it on a CDP.