Hey Christox,
(Sorry for my English aswell) I don't know if I'm going to be able to help you but I will try by telling you my experience with a Senn HD800 + HDVD800.
I too have a HD598 + Fiio E17. I really love the set. But I wanted better just like you, so I searched the internet for a good deal. I finally found the HD + HDVD800 set for 2100E wich is a pretty good deal. It was a demo set, so it was already burned in.
I did a lot of listening and my conclusion was that the dac section in the HDVD800 was not on par with the amp section, so the potential of the HD800 (wich I could't listen too using a better dac, in the store where I bought it, wich sounded unbelievably good) was lost.
The difference between my HD598 + Fiio E17 set and the HD + HDVD set was so small, that it simply wasn't worth the money for me.
So I came to the conclusion, if I want something that sounds really good (detail, soundstage, depth, holographics etc) I need a better dac which would be about 800Euro or more, I would need a balanced cable (some people don't believe in the better sq it can cause, I personally do believe) that cost about 200E. I wasn't satisfied with the fact the HD800 was damaged (demo model), so I would have bought that new. The price would become about 3200E or more. I believe that for that amount of money I could have something that is significantly better than what I already have and would stop my longing for better.
So I send the set back to the store and I am to this day enjoying my HD598 + FiioE17, with the knowledge that what I really want cost a lot of money and I would have to spend alot of money to get better sq.
Depending on the amount of money you have, I would recommend don't buy unless you can send it back (obviously) and don't expect too much, because your current set is already very good (especially for the money, I bought it for 300E).
If you have the money I would recommend HD800 + HDVA600 (analoge version) + (good dac) + balanced cable. If possible listen to something simular, so you know if you enjoy listening to it and what you do and don't like about it.
You have to try to make a set that has (almost) no bottleneck. Otherwise your throwing away money.
And in a HD700 + Fiio E17 set, the E17 will probably be a very big bottleneck.
Hope this helps and good luck with your quest
.