[s002wjh]

I have a b&o h6 and HD 280 pro + some other mostly less than 50ohm headphone.  mainly listening on PC/iphone6.  does the headphone benefit a lot from a external dac/amp such has oppo ha2se ?  or ill be just wasting money.  the realtek on my PC can sample up to 24bit/196khz.  the iphone 6 I think is 24bit/96khz.  I also don't listen music extremely loud.

 

[Weatherlight]

Short answer is yes. Way better sound quality.

 

[s002wjh]

well some say its better some say doesn't matter. provide the source/headphone is same, its hard for me to see improve audio quality between 24/196 vs 32/384 or even 16bit/96khz, as for amp clarity/loudness, the headphones are fairly easy to drive, so I wont have loudness issue.  amp clarity is another matter, a good amp does produce better clarity/etc vs onboard pc amp, just not sure human can tell the difference(unless they have goldenear, which I'm not one of them).  PC amp can be affected by EM within PC/iphone, but usually its very very small and a good PCB designer will fix that during design phase. 
 
hopefully someone did a comparison test would chip in.  if not then ill guess Ill just have to buy one and try it out.

 

[Raketen]

Being unable to tell between different bit depth/sampling rates doesnt mean you won't be able to appreciate a change in dac/amp, and AFAIK more than just being able to produce loudness there are numerous other properties that can affect audio and headphone performance perceptibly.

Still, differences can be relatively subtle- IMO if you are perfectly happy with your onboard audio probably don't NEED one (IIRC iPhones are reasonably well regarded for audio quality). That said, I think even my little Fiio m3 dap sounds better with more headphones than my laptop's Realtek based audio (which sounds just fine with some) - if you have disposable cash & listen to a lot of music, this particular can of worms might be worth a crack.