I tend to regard the M50 as the minimum for hi-fi listening with no apologies for limitations. But if you have to go cheaper than the usual $125 USD for the M50, the B&O Form2 with a simple bass boost gives a real hi-fi sound, and it should be available for about $100. The Beyer DTX-501P (similar to soundmagic P30) is good for $100, but needs a slight treble boost. The LSTN Fillmore with wood cups is somewhat colored, but still a good listen and good for $100. Below $100 are the Sennheiser PX series – not hi-fi by any means. There are several good IEMs below $100, and Apple Earpods with a Dirac or Accudio Pro player are very hi-fi, and cheap.
Case in point: the Koss PortaPro headphones first hit the market in 1984 and have become such a favorite with audiophiles that the company leaves the design (and the price tag) untouched. You can still pick one up for less than $50, and they come with a lifetime warranty, no receipt necessary. Check out our favorite budget headphones for more selections.
Electrostatic drivers consist of a thin, electrically charged diaphragm, typically a coated PET film membrane, suspended between two perforated metal plates (electrodes). The electrical sound signal is applied to the electrodes creating an electrical field; depending on the polarity of this field, the diaphragm is drawn towards one of the plates. Air is forced through the perforations; combined with a continuously changing electrical signal driving the membrane, a sound wave is generated. Electrostatic headphones are usually more expensive than moving-coil ones, and are comparatively uncommon. In addition, a special amplifier is required to amplify the signal to deflect the membrane, which often requires electrical potentials in the range of 100 to 1000 volts.
In-ear headphones are like earbuds but are an upgrade to them. These headphones fit into the wearer's ear canal and stay in place with the use of foam or rubber tips. These tips are available in custom sizes so they can fit each individual wearer. When a proper fit's achieved, in-ear headphones reduce outside noise and deliver sound quality at about the same level as over-ear and on-ear headphones.