I have not heard the PCD300a headphone output but I would definately not describe the PH100 as "not bad" in fact i think it is unbeatable at the price. The only reason I can think for it not being popular is it's price as even with the great build and sound quality it comes in at under $200.
At present the only headphones I would have reservations using with the PH100 are my Sony SA5000 which sound just a little too bright, other hedphones that work fine include the (very fussy) audio technica ath-w50000, denon ah-d5000, sennheiser hd650 and beyer dt990 pro.
The other amps I have are
Sugden Headmaster (x2) - not evryone's taste but definately mine!
Shanling PH300 (on its way to replace my second PH100)
Matrix (over rated - ph100 eats it alive)
Plus the following dacs with built in amps :-
matrix i-mini (dac excellent, headphone amp ok)
Yulong DAH1 (good but not recommended as I have had a couple 3 faulty items out of 4 ordered from yulong)
Aune (good basic amp but overrated)
Hilly (still listening)
My favourite source dacs would be the following
Xindak dac 5 and Keces 131.1 and 131 mk iii.
I have converted a few people to the PH100 by loaning them a unit for a couple of weeks!