@lewm.
Not only is it incorrect to assume that a headphone amplifier cannot also function as a preamp, but there also seems to be a misunderstanding regarding the roles of transformers and capacitors in the output stage.
While both transformers and capacitors can be used to block DC, capacitors are more commonly employed for this purpose because they are cost-effective and sufficiently effective. In contrast, output transformers are primarily used for impedance matching, voltage step-up or step-down, efficient current transfer, and providing isolation between stages.
Unlike tube preamps and power amplifiers, solid-state (SS) preamps and amplifiers typically have low output impedance and therefore do not require output transformers. As a result, most modern SS preamps are already output transformer-less (OTL).
The further elimination of capacitors in the output stage—leading to output capacitor-less (OCL) designs—is driven by the goal of improving transparency. These OCL designs instead rely on alternative methods, such as servo circuits (essentially feedback control), to manage and suppress DC offset as I mentioned earlier.