Short answer
Both are desktop amplifiers, $660 against $699.
The Grace Design m900 reaches 100 dB on 567 of the 567 models here, the Rupert Neve RNHP on 566.
| Specification | Grace Design m900 | Rupert Neve RNHP |
|---|---|---|
| Power into 32Ω | 1.0 W | 262 mW |
| Power into 150Ω | 228 mW | 175 mW |
| Power into 300Ω | 115 mW | 88 mW |
| Power into 600Ω | 57 mW | 44 mW |
| Output impedance | 0.08 Ω | 0.08 Ω |
| Output noise | 5.01 µV | 8.03 µV |
| Price | $660 | $699 |
| Headphones it reaches 100 dB on out of 567 | 567 | 566 |
| Load | Grace Design m900 | Rupert Neve RNHP |
|---|---|---|
| 32 Ω | Published | Derived |
| 150 Ω | Derived | Published |
| 300 Ω | Published | Derived |
| 600 Ω | Published | Derived |
Both makers publish a power rating. Grace Design m900 publishes at 32 Ω, at 300 Ω and at 600 Ω; Rupert Neve RNHP at 150 Ω. The other rows are worked out from those.
So the two columns are not equally firm row by row: at 32 Ω, at 300 Ω and at 600 Ω only the Grace Design m900 figure is the maker’s own and at 150 Ω only the Rupert Neve RNHP 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 Rupert Neve RNHP — Symphonium Titan.
88 headphones stay below the audible-hiss threshold on the Grace Design m900 but cross it on the Rupert Neve RNHP (64 Audio Aspire 1, 64 Audio Ocean, 64 Audio Volur, 85 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 +15 dB. 9 dB is about 8.3 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 | Rupert Neve RNHP headroom | Grace Design m900 hiss | Rupert Neve RNHP hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +44 dB | +35 dB | 28 dB | 33 dB |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +24 dB | +15 dB | 8 dB | 13 dB |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +28 dB | +25 dB | 7 dB | 11 dB |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +23 dB | +22 dB | 2 dB | 6 dB |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +22 dB | +15 dB | 1 dB | 5 dB |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +17 dB | +15 dB | -5 dB | -1 dB |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +12 dB | +11 dB | -9 dB | -5 dB |
| HiFiMAN Susvara | 60 Ω | 50 mW | +11 dB | +7 dB | -11 dB | -7 dB |
The Rupert Neve RNHP costs $39 more than the Grace Design m900, and on these three axes the money buys nothing: among the 567 models here there is not one that the Rupert Neve RNHP handles and the Grace Design m900 does not — not on power, not on the one-eighth rule, not on hiss. The traffic runs the other way: the cheaper Grace Design m900 drives one model to 100 dB and keeps 88 below the audible-hiss threshold that the Rupert Neve RNHP does not.
Only published figures count here; blank axes are counted separately.
Each in full: Grace Design m900 and Rupert Neve RNHP.