Headphones are really just small speakers. The amplifier that drives them is of paramount importance to how they sound.
Many CD players and pre-amps have headphone outputs on them. Generally speaking, the amplifier inside that is driving the headphone jack is a simple, cheap, opamp-type circuit with very little dynamic capability.
When using audiophile-type headphones with higher impedance, it takes a fairly high voltage level to get a reasonable volume. The cheap headphone amplifiers in CD players and preamps simply cannot generate the required voltage swing without clipping off the musical peaks.
A good tube amplifier has much more peak voltage capability than almost any solid-state amplifier. And it has the sonic characteristics of a tube amplifier, which many audiophiles find cannot be duplicated with solid-state equipment.
Wheatfield Audio's amplifiers are true tube amplifiers. Unlike some competitive products that claim to be a "tube amplifier" but use a solid-state follower for the output, or use an opamp input stage with tons of negative feedback to drive a tube output, the signal path in our amps is ALL tubes, and NO negative feedback..
For the tube purist, the HA-2 has no semiconductor devices in it - even the power supply uses a tube rectifier.