Short answer
Both are desktop amplifiers, $660 against $499.
The Grace Design m900 reaches 100 dB on 567 of the 567 models here, the Schiit Valhalla 3 on 566.
| Specification | Grace Design m900 | Schiit Valhalla 3 |
|---|---|---|
| Power into 32Ω | 1.0 W | 750 mW |
| Power into 150Ω | 228 mW | 286 mW |
| Power into 300Ω | 115 mW | 250 mW |
| Power into 600Ω | 57 mW | 125 mW |
| Output impedance | 0.08 Ω | 25 Ω |
| Output noise | 5.01 µV | 6.32 µV |
| Price | $660 | $499 |
| Headphones it reaches 100 dB on out of 567 | 567 | 566 |
| Load | Grace Design m900 | Schiit Valhalla 3 |
|---|---|---|
| 32 Ω | Published | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Published | Published |
| 600 Ω | Published | Derived |
Both makers publish a power rating. Grace Design m900 publishes at 32 Ω, at 300 Ω and at 600 Ω; Schiit Valhalla 3 at 32 Ω and at 300 Ω. The other rows are worked out from those.
So the two columns are not equally firm row by row: at 600 Ω only the Grace Design m900 figure is the maker’s own. On those rows the derived number is the one to doubt first.
1 headphone reaches 100 dB on the Grace Design m900 but not on the Schiit Valhalla 3 — Symphonium Titan.
535 headphones are left alone by the Grace Design m900 under the one-eighth rule but coloured by the Schiit Valhalla 3 (64 Audio Aspire 1, 64 Audio Fourte, 64 Audio Nio, 532 more).
41 headphones stay below the audible-hiss threshold on the Grace Design m900 but cross it on the Schiit Valhalla 3 (64 Audio Volur, AFUL Performer 5, Audeze Euclid, 38 more).
566 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: Grace Design m900 has +24 dB of headroom there against +17 dB. 7 dB is about 5.1 times the power, which matters for peaks rather than for how loud the music sits.
Hiss below about 20 dB SPL is inaudible in a quiet room.
| Headphone | Impedance | Needs | Grace Design m900 headroom | Schiit Valhalla 3 headroom | Grace Design m900 hiss | Schiit Valhalla 3 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +44 dB | +37 dB damping | 28 dB | 30 dB |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +24 dB | +17 dB damping | 8 dB | 10 dB |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +28 dB | +27 dB damping | 7 dB | 9 dB |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +23 dB | +26 dB | 2 dB | 4 dB |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +22 dB | +19 dB damping | 1 dB | 3 dB |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +17 dB | +20 dB | -5 dB | -3 dB |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +12 dB | +16 dB | -9 dB | -7 dB |
| HiFiMAN Susvara | 60 Ω | 50 mW | +11 dB | +10 dB damping | -11 dB | -9 dB |
The Grace Design m900 costs $161 more than the Schiit Valhalla 3. Out of the 567 models here, that difference buys 1 more model driven to 100 dB, 535 more models left alone under the one-eighth rule and 41 more models kept below the audible-hiss threshold.
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 in full: Grace Design m900 and Schiit Valhalla 3.