Both are Bluetooth receivers, $90 against $109. This page puts them side by side on the only three things a maker can be held to — power, output impedance and noise — and then asks the question that actually matters: which headphones do they treat differently?
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 |
A figure marked not published is missing from the maker’s own material, which is not the same as being good or bad. Where one of these two publishes a number and the other does not, the comparison on that axis cannot be made at all — and saying so is more useful than guessing.
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. That is a power count and nothing more; whether either source suits a given headphone also depends on the two lines above.
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. Or put either against your own headphones in the source comparison tool.