Yes I will too report that the headphone output on the DAC1 is a bit...bright and unrefined, almost sharp, so to speak, it's not bad, but it takes away from the DAC section and kinda ruins it. I wouldn't use it as a one unit and would go for a separate headphone amp to make use of the DAC to the full extent. I much prefer the headphone output of the Asus Essence One Mk2 Muses Edition. Both devices cost roughly the same, but Asus has a more powerful headphone output and a much better sounding one, it just has an amazingly pleasing timbre with those Muses 01 op-amps, it sounds very smooth and full bodied, but at the same time extremely detailed, spacious, fast, just excellent micro-dynamics. It also has a massive soundstage, a word "holographic" easily applies to it, it just makes things sound palpable and closer to being in front of you than with the DAC1. It's superior to any separate headphone amp under 500-600 dollars I've heard. Easily beats the Schiit Asgard 2 and Musical Fidelity M1HPAP for example. The comparison was done with the T1's and HD600's. I haven't tried both devices just as DAC's side by side, only with their headphone outs. Teac UD-503 is another beast of a DAC in this price category, probably the best of the three when it comes to being a standalone DAC, but it has a slightly weaker headphone output, but still good enough to work as a single unit with high end headphones. However, it has the ability to be a balanced headphone amplifier, that makes the amp better, so if you have balanced headphones, it might be the best as an amp too. It also looks and feels epic, Asus is built like a brick, it feels like it's glued to the table when I try to pick it up, that's how heavy it feels for it's size, but it still has plastic knobs that feel kinda cheap. Marantz looks a bit kitschy with that fake wood on the sides, and also has cheap feeling knobs, but the Teac is built like a tank, all the knobs are metal and super solid and precise, it just feels like a high end piece of equipment. Lots of good choices at the moment when it comes to all-in-one DAC/head amp units. I haven't tried the Naim DAC-V1, but that's slightly out of my budget.
