Short answer
Both are Bluetooth receivers, $90 against $109.
The FiiO BTR5 2021 reaches 100 dB on 563 of the 567 models here, the EarStudio ES100 MK2 on 553.
| Specification | EarStudio ES100 MK2 | FiiO BTR5 2021 |
|---|---|---|
| Power into 32Ω | 60 mW | 80 mW |
| Power into 150Ω | 20 mW | 17 mW |
| Power into 300Ω | 10.0 mW | 8.5 mW |
| Power into 600Ω | 5.0 mW | 4.3 mW |
| Output impedance | 0.5 Ω unverified | 1.0 Ω |
| Output noise | 3 µV unverified | 2.0 µV |
| Price | $90 | $109 |
| Headphones it reaches 100 dB on out of 567 | 553 | 563 |
| Balanced | EarStudio ES100 MK2 | FiiO BTR5 2021 |
|---|---|---|
| Socket | no separate rating | 2.5mm |
| Power into 32Ω | — | 240 mW |
| Power into 300Ω | — | 26 mW derived |
| Output impedance | — | 2.0 Ω |
| Output noise | — | 2.2 µV |
| Headphones it reaches 100 dB on out of 567 | — | 565 |
Only the FiiO BTR5 2021 has a separate balanced rating from its maker; through it, it reaches 100 dB on 565 of the 567 models here. The EarStudio ES100 MK2 may still have a balanced socket — its spec sheet simply gives no figure for one, so there is nothing to put beside it. Derived figures are worked out from the loads the maker did publish.
| Load | EarStudio ES100 MK2 | FiiO BTR5 2021 |
|---|---|---|
| 32 Ω | Unverified | Published |
| 150 Ω | Unverified | Derived |
| 300 Ω | Unverified | Derived |
| 600 Ω | Unverified | Derived |
EarStudio ES100 MK2 publishes no electrical specification at all. That is true of 10 of the 280 sources here, and the figures in that column come from independent measurement rather than from a maker document — which is not worse, but it is not the same thing, and it cannot be checked against anything the maker will stand behind.
The FiiO BTR5 2021 column does stand on a power rating its maker printed, at one of these four loads at least. Reading the two straight across sets one kind of claim against another, and nobody has written down the conditions of both side by side.
10 headphones reach 100 dB on the FiiO BTR5 2021 but not on the EarStudio ES100 MK2 — HiFiMAN HE-6se V2, HiFiMAN HE6 Remastered, Moondrop COSMO, 7 more.
553 of the 567 models here reach 100 dB on both.
Headroom is at a 100 dB SPL peak.
The widest gap on the list is the Shure SE846: FiiO BTR5 2021 has +31 dB of headroom there against +26 dB. 5 dB is about 3 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for EarStudio ES100 MK2 because that maker publishes no output noise figure.
| Headphone | Impedance | Needs | EarStudio ES100 MK2 headroom | FiiO BTR5 2021 headroom | EarStudio ES100 MK2 hiss | FiiO BTR5 2021 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +26 dB | +31 dB | not published | 20 dB |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +6 dB | +11 dB | not published | 0 dB |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +16 dB | +17 dB | not published | -1 dB |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +13 dB | +12 dB | not published | -6 dB |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +8 dB | +10 dB | not published | -7 dB |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +6 dB | +5 dB | not published | -13 dB |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +2 dB | +1 dB | not published | -17 dB |
| HiFiMAN Susvara | 60 Ω | 50 mW | -1 dB | -1 dB | not published | -19 dB |
The FiiO BTR5 2021 costs $19 more than the EarStudio ES100 MK2. Out of the 567 models here, that difference buys 10 more models driven to 100 dB.
Nothing runs the other way: there is no axis here on which the cheaper of the two does better.
Only published figures count here; blank axes are counted separately.
Each has a page of its own with a full headphone list and the provenance of every figure — worth opening here, because EarStudio ES100 MK2 has published no electrical specification at all and everything above comes from independent measurement: EarStudio ES100 MK2 and FiiO BTR5 2021.