Short answer
Both are portable players, $799 against $999.
The FiiO M15S reaches 100 dB on 566 of the 567 models here, the HiBy RS6 on 564.
| Specification | FiiO M15S | HiBy RS6 |
|---|---|---|
| Power into 32Ω | 580 mW | 180 mW |
| Power into 150Ω | 150 mW | 38 mW |
| Power into 300Ω | 93 mW | 19 mW |
| Power into 600Ω | 47 mW | 9.6 mW |
| Output impedance | 1.4 Ω | 0.4 Ω |
| Output noise | 3.2 µV | not published |
| Price | $799 | $999 |
| Headphones it reaches 100 dB on out of 567 | 566 | 564 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Balanced | FiiO M15S | HiBy RS6 |
|---|---|---|
| Socket | 4.4mm (2.5mm also provided) | 4.4mm |
| Power into 32Ω | 1.2 W | 690 mW |
| Power into 300Ω | 368 mW | 74 mW derived |
| Output impedance | 2.3 Ω | 0.8 Ω |
| Output noise | 5.2 µV | not published |
| Headphones it reaches 100 dB on out of 567 | 567 | 566 |
Through the balanced sockets the FiiO M15S reaches 100 dB on 567 of the 567 models here and the HiBy RS6 on 566. Derived figures are worked out from the loads the maker did publish.
| Load | FiiO M15S | HiBy RS6 |
|---|---|---|
| 32 Ω | Published | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Published | Derived |
| 600 Ω | Derived | Derived |
Both makers publish a power rating. FiiO M15S publishes at 32 Ω and at 300 Ω; HiBy RS6 at 32 Ω. The other rows are worked out from those.
So the two columns are not equally firm row by row: at 300 Ω only the FiiO M15S figure is the maker’s own. On those rows the derived number is the one to doubt first.
2 headphones reach 100 dB on the FiiO M15S but not on the HiBy RS6 — Moondrop PARA, Symphonium Crimson.
50 headphones are left alone by the HiBy RS6 under the one-eighth rule but coloured by the FiiO M15S (64 Audio Aspire 1, 64 Audio Fourte, 64 Audio Nio, 47 more).
564 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 +11 dB. 8 dB is about 5.9 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for HiBy RS6 because that maker publishes no output noise figure.
| Headphone | Impedance | Needs | FiiO M15S headroom | HiBy RS6 headroom | FiiO M15S hiss | HiBy RS6 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +39 dB damping | +31 dB | 25 dB | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +19 dB damping | +11 dB | 5 dB | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +26 dB | +20 dB | 3 dB | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +22 dB | +16 dB | -2 dB | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +18 dB | +12 dB | -3 dB | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +16 dB | +9 dB | -9 dB | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +11 dB | +5 dB | -13 dB | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +8 dB | +3 dB | -15 dB | not published |
The HiBy RS6 costs $200 more than the FiiO M15S. Out of the 567 models here, that difference buys 50 more models left alone under the one-eighth rule.
It does not run one way only: the cheaper FiiO M15S drives 2 models to 100 dB that the HiBy RS6 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.