Impressions of Unamped Highend headphones will be tough to find, cause after spending so much on the HP its only natural that one spends on Amp/DAC.
That being said , you can ask the first hand impressions of members here,
I have ATH-W5000, but i cant seem to find 1/4">>1/8" adapter, ill post impressions of W5000 driven out of sony walkman , when i find the adapter.
Benny can tell you how M50 and D5000 sound unamped ( is it worth spending on , for unamped use ) . ampretty sure the Sennheiser HD25 is not good unamped .
remember W5000 , AD900, AD2000, D5000, D7000 are HUGE headphones , not practical for portable use. almost impossible to carry in laptop bags. you will have to get specific bag for then if you want to carry them
M50 , Ultrasone signature pro, (and ultrasone pro series ) are much better for portable use cause they have fold-able design