Mind you, dCS use both linear mode and switch-mode amplification.
Anyway in the Bartók headphone DAC:
Amplification for headphones
In the past, it has been common for dCS owners to use the existing line out stage on their unit to drive headphones, by making up a special cable to run from the outputs to the headphones. It actually does this job reasonably well – even though not designed for the purpose – with its low output impedance and low harmonic content and distortion, and its ability to drive a fair amount of current. When we designed the Bartók Headphone DAC, however, we decided to add a specially-designed headphone amplifier to drive a wider range of headphones (particularly low-impedance types, which wouldn’t do so well on the existing line output), both balanced and unbalanced, directly, in addition to the standard line outputs. The design, which attempts to retain the essential character of the existing line output stage while adding the functionality described above, follows a dual-mono configuration, keeping the two channels completely separate. The amplifiers are powered by a combination of linear and switch-mode power supplies that are completely separate from those employed in other parts of the product, particularly necessary as quite high ground return currents are experienced with low-impedance headphones and these need to be kept
well clear of the DAC and associated circuitry. A substantial toroidal transformer is employed in the linear PSU.
From: [page 11]: