But unlike loudspeakers, headphones usually do have very flat impedance curves and they offer benign and relatively high-impedance, tube-friendly loads (30-300 ohm loads are typical for headphones, rather than the 4-8 ohm loads typically seen with loudspeakers). In practice this means that for the most part impedance matching issues between tube-type headphone amps and headphones don’t come into play—at least not to the dramatic degree that they do between tube-type power amps and speakers. (If one wants an analogy to traditional full-size audio system, it is probably better to think of headphone amps as behaving more like preamplifiers than like power amps.). The bottom line, then, is that many headphone aficionados regard tube-type amps as the solution of choice for powering almost all kinds of high-quality headphones.
Second, it is important to bear in mind that headphones behave much differently than speakers do, from the listener’s point of view. In particular, the performance of speaker-based audio systems is governed—or at least heavily influenced—by the speaker-room interface. But for obvious reasons, the “speaker-room interface” isn’t part of the deal in headphones. My experience in reviewing many headphones is that the absence of room interface issues makes frequency response errors and distortions in headphone-based systems much easier to identify and to focus on. This is fine up to a point, but it also exposes one of the