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
Except for the M50 and the Hd25-1 II, none of the headphones mentioned are portable.
Unamped the m50 sounds just about right with the sflo2. I don't think the iphone or any other phone or an ipod can drive it as good as the sflo2. Amped, they would sound much more fuller and precise. Same for the d5000, unamped, it may sound loud at high volumes but doesn't sound full. Unamped D5k is in no way the right way to listen to this headphone. It is a waste of something really good, something which is meant to be cherished with a decent dac/amp.