I'm very much enjoying the sound quality of the iDSD. Very resolving, great soundstage, (although not terribly wide), and for the most part (more below) a black background. This is with redbook-level files, using either Music Bee or Tidal, and with my HD600s. This is actually a very good pairing, as the 600s have enough detail retrieval to take advantage of the same characteristic of the iDSD. Same with the soundstage and instrument location in space,(especially with the 3D on).
One anomaly though. On two different occasions now, I've been able to hear some very quiet clicks and pops in the background during and between songs. It sounded very similar to an old vinyl album ironically. I don't think its the same thing as others have reported because I don't have any DSD tracks (yet, I'll probably try at least one or two albums). Each time, it has been present no matter the settings of the iDSD. The first time it happened with Tidal, but when I switched to my collection, no noise. I assumed it was more teething pains with Tidal (which I love overall). The second time was last night, and was present with any source. When I noticed it I was trying out some iems (w40, DJ-E1500, XBA-H100) for the first time with the iDSD, but it happened with the 600s as well. No noise today, so keeping my fingers crossed. I was very tired and decidedly unsober last night though, so who knows? It does work well and sound great w/ iems btw, but with this much power on tap (with most sources I can drive the 600s to listening volume on normal power mode at 12-2 o'clock), I'm going to have to remember to be careful with the settings so I don't deafen myself or send a set of iems flying out of my ears.
I hated the 3d mode with the iDSD in preamp mode feeding a receiver and speakers through the analog outs. It created some weird artifacts and sounded very unatural in this totally lo-fi set-up, but that's not what I got it for anyway. I'm still learning its sound and features, but so far, barring a repeat of the noise, two thumbs up.