High sensitivity headphones are great because they don't require a lot power to drive them but the downside to sensitive headphones is that they tend to produce audible background hiss that transfers into the audio you're playing. How much hiss you get depends on the audio source the headphone is connected to (i.e. smartphone, digital audio player, single-ended triode amplifier, etc).
One way to solve the background hissing (or humming) you get with high sensitivity headphones is by increasing their output impedance, which works great with single driver IEMs but not so good with IEMs that have multiple balanced armature (BA) drivers because each driver has different impedance so, if the output impedance of the audio source is higher than zero, you can get a change in frequency response (i.e. lower bass response).
The easier way to solve background hiss and imprecise control of volume is with an attenuator like Ear Buddy and iEMatch, which are audio attenuators that reduce the output of a headphone amplifier to minimize channel imbalances at low volume and background hiss. Both Ear Buddy and iEMatch are practically the same product but iEMatch lets you adjust the output impedance and can be used for single ended and balanced connections.
If you were to plug a pair of IEMs such as the EarStudio HE100, KZ ZS10 Pro or Brainwavz Delta, which have high sensitivity (i.e. above 100dB) and low impedance (i.e. below 20 ohms) into a single-ended triode (SET) tube amplifier (i.e. ALO's Studio Six) or any amp that is not intended for IEMs, you immediately will get a lot of background hiss. With iFi Audio's Ear Buddy or iEMatch, it makes tube amps and any amp that is not intended for IEMs viable for IEMs as it reduces background hiss so, it is no longer audible.
Ear Buddy and iEMatch smooth out the audio, making the signal cleaner, while reducing the volume in the process, which is not a problem with high sensitivity IEMs (i.e. above 115dB) plugged into an amp with volume gain. But, if you were to plug low sensitivity IEMs (i.e. below 80db) into the iEMatch 3.5mm, using the 3.5 mm headphone jack of a phone the volume won't be loud enough because the volume level is attenuated by the audio attenuator, making low sensitivity IEMs even quieter.
There are a total of four adjustment settings on the iFi iEMatch, which are controlled via two physical switches. One of those switches is located on the 3.5mm female plug end of iEMatch, while the other switch is located on the gold plated 3.5mm male L-plug connector, which lets you select between balanced (BAL) headphone with TRRS and single ended (SE) headphone with TRS. You can also plug in a headset but the microphone gets disabled.
The settings on the female socket lets you adjust headphone sensitivity from high-gain (2.5 ohm-load) for high sensitivity headphones (i.e. 115db) and ultra (1 ohm-load) for very high sensitivity headphones (i.e. 130dB). The iFi iEMatch ultra-load reduces volume by 24dB, while the high-load reduces volume by 12dB.
The iFI iEMatch body shell is made of 6063-T5 aluminum with magnesium alloy so, iEMatch is strong and lightweight, weighing just 13 grams. The wiring is made of 6N silver-copper metal matrix composite protected by a transparent sleeve. Total length is 12cm long.
The iFi Ear Buddy shell is made of rubberized hard plastic so, it only weighs 6 grams. The wiring is of 6N copper matrix protected by a flat rubber orange sleeve. Total length is 12 cm long.
There is no headphone sensitivity adjustment so, the Ear Buddy only features a default high load setting that attenuates the volume by 16dB. The iFi Ear Buddy does have the same gold-plated printed circuit board you get with the iEMatch, as well as a gold-plated 3.5mm male connector and gold plated female socket.
The Ear Buddy input impedance is the same as the iEMatch (16 Ohm) but has a source impedance of just 18 Ohm and a fixed output impedance load of 2.7 Ohms; hence you will experience the same "quieter volume" when plugging Ear Buddy to a smartphone. Both Ear Buddy and iEMatch support headphones with impedance rating from 16 Ohm to 600 Ohm (12.5 Ohm is the minimum impedance supported).
Both Ear Buddy and iEMatch use two resistors instead of one resistor, which would hugely increase the output impedance. This means if you plug Ear Buddy and iEMatch into a headphone amplifier with high output impedance, you reduce the high impedance of the headphone amplifier and correct the impedance mismatch of a pair of sensitive headphones
Included with both Ear Buddy and iEMatch are a pair soft -37dB earplugs, a gold plated airplane two-prong adapter, hard-shell zip case (Ear Buddy) and a suede-like drawstring pouch (iEMatch. There is also a 12 month warranty for both products but you must register online at iFi Audio.