I listened to the dCS Bartok Apex and its SE headphone output for about half an hour today. Standard DXD upsampling was used with Filter 3 and Mapper 3. The headphone I used was my new Sony MDR-Z1R. I played around with some of the settings, but the differences between the filters were too subtle to detect within a few seconds, and I wanted to spend more time listening to the unit rather than just the settings.
Compared to the Sony Walkman ZX707 with DSD Remastering enabled, the Bartok is more open sounding, with slightly more depth to the soundstage. It sounds both more neutral and richer, with less of the direct bass emphasis/boost of the ZX707, but also more richness to the tone of notes. It somewhat paradoxically sounded both smoother and more detailed. I could hear that the leading edges of notes, the transients, were softer and smoother than the sharper attack presented by the Walkman, but at the same time I seemed to have an easier time picking out background notes and sound cues. Perhaps the increased staging and openness played a role. The result is that the Bartok overall gave a more relaxed and open presentation while the Walkman presented a more energetic, forward, and bassy+sparkly sound. The Bartok's openness was great for classical music in combination with the innate spatial qualities of the Z1R, and I thoroughly enjoyed Beethoven's Symphony No. 5 on it.
I'm not in the market for a Bartok right now, and I probably won't be for the foreseeable future as it's just so expensive, but I can see the appeal. I didn't think it was overly smooth, but it certainly was smooth and rich without making the listener feel like they were missing details due to that smoothness. On the other hand, I can see this sound not being optimal with other headphones. I was unmoved by the Rossini Apex feeding my Stax amp + Shangri-La Jr electrostatic setup. On that amp and headphones (which are already very smooth amongst electrostatics), I really enjoyed the dynamic and punchy sound of the Ferrum Erco while the smooth richness of the Rossini didn't do it many favors. The Z1R has the capacity for sharpness and harshness that can be mellowed by the Bartok.