Rudolfs Putnins of Headphonesty left the tube guards off his Feliks Audio Euforia Evo because they got in the way of "getting hypnotized by the bottled fire." That glow is close to the music in a Feliks amp. Most tube amplifiers put a transformer between the tubes and the headphones to make the two get along. The Echo, Elise and Euforia leave it out, a design called output-transformerless, or OTL, so the tubes connect almost directly to the headphones. Putnins notes the argument that dropping the transformer lets a listener hear more of the tubes themselves. The family aims for a neutral sound, Łukasz Feliks says, with as few parts in the music's path as possible. The tradeoff is that these amps prefer headphones that ask for voltage rather than current, and George Dobrescu of Audiophile Heaven heard a brighter sound with less bass when he used the wrong kind. Feliks suits listeners with classic dynamic headphones, such as Sennheiser's HD 600 family, who want tube texture with a neutral balance.

The Echo is the place to start, and it shines with a Sennheiser. Stephen Kostas of Bloom Audio called the Echo MKII's midrange "absolutely off the charts" and thought it and an HD650 might be a "mid-fi endgame." Dobrescu recommends headphones rated from 80 to 600 ohms for it. The Elise is the step up and the tube roller's model. Aornic found it sensitive to changes of tubes, and the channel suggests 6080 power tubes for rock and metal and the stock Svetlana tubes for a smoother tone. At the top of the OTL range, Putnins called the Euforia Evo "the least compromised OTL tube headphone amplifier I've heard." If you own hard-to-drive planar headphones such as the HE6 or Susvara, skip the OTL models and look at the Envy, the first Feliks headphone amp with output transformers. Matthew of Ear Fidelity found it had "more than enough power" for the Susvara. It runs warm, so HiFi Pig suggests a well-ventilated spot.

Henryk Feliks, an electronics engineer, built tube amplifiers for himself, then for friends and local bands, and in the 1990s the hobby became a small business, his son Łukasz told Marek Dyba of HiFi Knights. The Feliks-Audio brand began about 20 years before that 2020 interview, with a small OTL headphone amp called the Espressivo. Henryk's three sons joined in 2014. Łukasz handles sales, Piotr supervises production, Michał leads design and marketing, and their father concentrates on new products. Łukasz credits the Elise, designed with steady feedback from members of Head-Fi.org, with making the brand's name outside Poland. The amps are assembled by hand in Lubliniec. Feliks builds speaker amps as well. Steve Guttenberg asked to review the Arioso 300B, a single-ended triode amp, after his friend Herb Reichert of Stereophile kept raving about a Feliks headphone amp, and with Klipsch Cornwall IVs he found himself listening "way into the night." Łukasz says the Euforia served for a time as Focal's reference tube amplifier.