My experience with all headphone is that once you get into a decent DAC (AQ Dragonfly, Meridian Explorer, CEnterence DACport, etc) you are at the 90-95% of the sound quality you are likely going to get. e.g. you are into the world of diminishing returns where you typically spend ever increasing amounts of money to get small improvement in sound quality.
The Oppo 105 is an amazing versatile item. If you need that versatility then you won't regret using the Oppo 105, and would likely need to spend close to the price of the Oppo 105 on a dedicated DAC to make a significant improvement in SQ. I would say the Oppo 105 is not going to stop you from enjoying the HD800. Something like the Master-7 or PWD would be a step up. How much of an improve you hear I can't predict, but I doubt it will be a day -vs- night difference.
With amplifiers you want an amplifier than can cleanly put 500mw into the HD800 or you will be seriously holding it back. This rules out all the USB only DACs or low power headphone out/amplifiers I have tried. But once you have gotten into on the the decent amplifiers you will be at the 90% of the SQ and the HD800 will sound quite good.
The HD800 scale very well, so going with a top end headphone amplifier will improve the sound quality. I found the HD800 very enjoyable with a DragonFly into a Headamp Pico Power or GS-1, or being driven my the HiFi-M8. But a top end DAC + the Headamp GS-Xmk2 did take the SQ up a level. Personally I don't think there is a better amp for the HD800 than the GS-Xmk2, though there are a few which are different but equal such as the EC Balancing Act.
Whatever you settle on, just remember that most of us experience adaptive hedonism, e.g. what is amazing the first time we hear it can easily become just ok after awhile because you get used to what is amazing. This can lead to a never ending chase (and emptying of the wallet). Personally, I recommend people find a sound quality that lets them enjoy the music and doesn't distract and then work to cultivate a sense of gratitude for what they are hearing.
--Mark