I finally had some time with HD800s on iDSD. But unfortunately, for some other commitments, I ended up bringing iDSD to my office, and R26 back to home. So the following impression is based on iDSD → HD800s (C1) and iDSD → Burson Soloist 3XP → HD800s (C2). I will bring iDSD back home eventually and will come back here to give an update. Anyway, my thoughts on the aforementioned pairs.
The verdict comes first: I prefer C2, by a small but meaningful margin.
They both have ample power for HD800s. On middle gain with the volume set to about 40-45% on iDSD and 57 on 3XP, Hd800s got to a similar decimal on two chains. The first apparent difference is that the whole presentation, particularly the mids are more forward on C1, more laid-back and relaxed on C2. It is also noticeable that C2 had better control of mid-high frequencies. The details and resolution on both chains are very similar, but it sounds more polite, effortless and generally more pleasant on C2, whereas on C1, HD800s sounds more exciting and energetic but can be a tad shouty on some tracks. The bass is very similar on both chains, clean, fast not too much in quantity, this is a HD800s after all, you know the drill. And finally, because of the laid-back presentation, the sounds stage and the overall sense of space are definitely bigger on C2, but for some modern vocal-centric tracks, I actually prefer C1's intimate presentation, because HD800s already had a very wide sometimes spreading character, to balance it out definitely helps in tracks where you suppose to hear singers whispering into your ears.
To me, the overly bright and sometimes hissing highs on HD800s is the ONE thing I do not like about this can. So I definitely prefer a more controlled and laid-back presentation on C2. But if you like a mid-forward style of sound, then iDSD is definitely more fitting. That's my 2 cents on this. Mind me that these are very short and early impressions. I will update here if my mind changes along the way.