As usual in audio rules had many exceptions...
I tried to upgrade for my K340 from my integrated 100 watts Sansui alpha... I buy one of the best tubes amplifier dedicated for headphone there is...
There was more apparently at first 15 minutes with this tube headphone amp. more details and there was certainly more silent background noise for sure even if the Sansui was very low noise already ...
But there is many acoustic factors in sound... Almost none were better on the upgraded celebrated headphone amplifier ... SYNERGY matter more than any rule...With the Sansui the K340 are stellar top heaphone ... With the upgraded headphone tube amp : artificial headphone timbre sound , no more out of the head impression and no 3-D holography with a natural realistic speaker like sound ...
I returned for a refund where i lost much money ( post fees customs insurance for import and export shipment ) ....😊
My K340 is so picky almost no headphone amp will serve them better that the Sansui alpha paid 300 bucks 8 years ago ...
Then if you had "an headphone out" from a vintage top flagship amp. of many decade ago and if you are stunned by the sound dont "upgrade" ...😁
My actual modified active speakers alimented by the right dac in their acoustic corner are low cost and beat most headphones easily in nearfield listening ... My k340 beat them in all aspects and are my dedicated system for dedicated listening... My astonishing small speakers are for casual listenings many hours each day...
my system value is 700 bucks... I am proud of it as many others are proud of their costlier system... In a race for quality i am no so much far behind that what most people think ...
my headphone and their amp is vintage flagship of yesterday but yet always top product among today offerings ... Not the best there is... The best in the ratio price /S.Q. scale ...
Then some headphone out can beat dedicated amplification ...