Short answer
Both are portable players, $800 against $799.
The FiiO M15S reaches 100 dB on 566 of the 567 models here, the Astell&Kern SR35 on 565.
| Specification | Astell&Kern SR35 | FiiO M15S |
|---|---|---|
| Power into 32Ω | 300 mW | 580 mW |
| Power into 150Ω | 64 mW | 150 mW |
| Power into 300Ω | 32 mW | 93 mW |
| Power into 600Ω | 16 mW | 47 mW |
| Output impedance | 1.1 Ω | 1.4 Ω |
| Output noise | 4 µV unverified | 3.2 µV |
| Price | $800 | $799 |
| Headphones it reaches 100 dB on out of 567 | 565 | 566 |
| Balanced | Astell&Kern SR35 | FiiO M15S |
|---|---|---|
| Socket | 4.4mm and 2.5mm | 4.4mm (2.5mm also provided) |
| Power into 32Ω | 1.1 W derived | 1.2 W |
| Power into 300Ω | 120 mW derived | 368 mW |
| Output impedance | 2.7 Ω | 2.3 Ω |
| Output noise | not published | 5.2 µV |
| Headphones it reaches 100 dB on out of 567 | 567 | 567 |
Through the balanced sockets the two are level on the same test: each reaches 100 dB on 567 of the 567 models here. Derived figures are worked out from the loads the maker did publish.
| Load | Astell&Kern SR35 | FiiO M15S |
|---|---|---|
| 32 Ω | Upper bound | Published |
| 150 Ω | Upper bound | Derived |
| 300 Ω | Upper bound | Published |
| 600 Ω | Upper bound | Derived |
These two columns are not the same kind of number, and that matters more than the gap between them. FiiO M15S publishes a power rating (at 32 Ω and at 300 Ω); Astell&Kern SR35 publishes no power figure at any load, only a maximum output voltage, so every milliwatt in its column is a ceiling worked out from that voltage.
A ceiling assumes the amplifier holds that voltage into the load, and nothing on Astell&Kern SR35’s own sheet says it does. It is the most flattering reading available, and it is used here only because the alternative is a blank. Where the Astell&Kern SR35 column looks the stronger of the two, treat the difference as unproven rather than measured. 24 of the 280 sources here are in that position, with no published power rating at any load.
1 headphone reaches 100 dB on the FiiO M15S but not on the Astell&Kern SR35 — Moondrop PARA.
23 headphones are left alone by the Astell&Kern SR35 under the one-eighth rule but coloured by the FiiO M15S (64 Audio Aspire 1, 64 Audio Fourte, 64 Audio Ocean, 20 more).
565 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 Campfire Audio Andromeda 2020: FiiO M15S has +19 dB of headroom there against +13 dB. 6 dB is about 3.6 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for Astell&Kern SR35 because that maker publishes no output noise figure.
| Headphone | Impedance | Needs | Astell&Kern SR35 headroom | FiiO M15S headroom | Astell&Kern SR35 hiss | FiiO M15S hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +33 dB | +39 dB damping | not published | 25 dB |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +13 dB | +19 dB damping | not published | 5 dB |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +22 dB | +26 dB | not published | 3 dB |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +18 dB | +22 dB | not published | -2 dB |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +15 dB | +18 dB | not published | -3 dB |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +11 dB | +16 dB | not published | -9 dB |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +7 dB | +11 dB | not published | -13 dB |
| HiFiMAN Susvara | 60 Ω | 50 mW | +5 dB | +8 dB | not published | -15 dB |
The Astell&Kern SR35 costs $1 more than the FiiO M15S. Out of the 567 models here, that difference buys 23 more models left alone under the one-eighth rule.
It does not run one way only: the cheaper FiiO M15S drives one model to 100 dB that the Astell&Kern SR35 does not, which is the part worth checking against your own headphones before paying the difference.
Only published figures count here; blank axes are counted separately.
Each in full: Astell&Kern SR35 and FiiO M15S.