The Sennheiser HD 600 is 300Ω at 91.8 dB/mW; the Sennheiser HD 660S is 150Ω at 95.8 dB/mW. This page is only about what they ask of an amplifier — not how they sound, which no specification on either sheet can tell you.
The HD 600 needs 4.0 dB more than the HD 660S to reach the same loudness — 6.6 mW against 2.6 mW.
| Specification | HD 600 | HD 660S |
|---|---|---|
| Impedance | 300 Ω | 150 Ω |
| Sensitivity | 91.8 dB/mW | 95.8 dB/mW |
| Sensitivity (voltage) | 97.0 dB/V | 104.0 dB/V |
| Power for 100 dB | 6.6 mW | 2.6 mW |
| Voltage for 100 dB | 1.41 V | 0.63 V |
| Current for 100 dB | 4.7 mA | 4.2 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 37.50 Ω | 18.75 Ω |
| Sources that reach 100 dB out of 280 | 261 | 280 |
19 of the 280 sources have the power for the HD 660S but not for the HD 600 — Apple USB-C dongle (US), Apple USB-C dongle (EU), JadeAudio JA11, 16 more.
2 sources are low enough in output impedance for the HD 600 under the one-eighth rule but not for the HD 660S (Schiit Valhalla 3, Arturia AudioFuse Rev2).
261 of the 280 sources here reach 100 dB on both. That count is about power only — the output-impedance and hiss lines above are separate questions, and a source can pass this one and fail those.
The gap between these two in decibels does not change with volume — it is a property of the two headphones. What changes is the size of the numbers, and it changes fast: every 10 dB costs ten times the power. The row that matters is the one nearest the level you listen at, and for most people that is nearer the top of this table than the bottom.
The HD 600 is more demanding than 99% of the 567 non-electrostatic models in this database; the HD 660S than 91%. Both figures use the same measure as the rest of the site: whichever runs out first against a 2 V, 100 mW dongle, voltage or power.
| dB SPL | HD 600 power | HD 600 voltage | HD 660S power | HD 660S voltage |
|---|---|---|---|---|
| 80 | 0.07 mW | 0.14 V | 0.03 mW | 63 mV |
| 85 | 0.21 mW | 0.25 V | 0.08 mW | 0.11 V |
| 90 | 0.66 mW | 0.45 V | 0.26 mW | 0.20 V |
| 95 | 2.1 mW | 0.79 V | 0.83 mW | 0.35 V |
| 100 | 6.6 mW | 1.41 V | 2.6 mW | 0.63 V |
| 105 | 21 mW | 2.50 V | 8.3 mW | 1.12 V |
105 dB is included because forum power figures are usually quoted there or higher. It is also a level that damages hearing in minutes — how many minutes is its own calculator.
| Source | Price | Headroom, HD 600 | Headroom, HD 660S | Output impedance | Noise |
|---|---|---|---|---|---|
| FiiO KA11 | $29 | 5.2 dB | 12.2 dB | 0.7 Ω | 1.7 µV |
| FiiO BTR13 | $59 | 1.8 dB | 9.0 dB | 0.6 Ω | 1.0 µV |
| iBasso DC03 Pro | $69 | 2.0 dB | 9.0 dB | 0.12 Ω | 0.9 µV |
Of the 280 sources here, 261 reach 100 dB on both. That is the easy axis. 68 of them also publish an output impedance low enough for both under the one-eighth rule and a noise figure that stays below the audible-hiss threshold with both — the rest either fail one of those or never printed the number at all, and 192 of them are in the second group.
The cheapest of the 68 is the FiiO KA11, at $29. It leaves 5.2 dB of headroom on the HD 600 and 12.2 dB on the HD 660S, its output impedance is 0.7 Ω against a one-eighth limit of 37.50 Ω and 18.75 Ω, and its noise figure puts the hiss floor 38 dB and 31 dB below the 20 dB SPL this site treats as the point where a quiet room stops hiding it.
Cheapest that satisfies three published numbers is not the same as best, and none of it is about how either headphone sounds. To put either model against a source this page does not mention, or to change the target loudness, use the amplifier matcher.
| Model | As printed | Tolerance | Standard |
|---|---|---|---|
| HD 600 | 12 Hz - 40,500 Hz | not printed | not printed |
| HD 660S | nothing printed | — | — |
Sennheiser prints 12 Hz - 40,500 Hz for the HD 600 and nothing at all for the HD 660S.
HD 660S: Sennheiser still has a page for the HD 660 S, but it is a retired-product page: it offers a replacement model and spare parts and carries no specification table at all. The same site prints a full table, frequency response included, for the models it still sells.
So one side of this comparison has a frequency-response number and the other does not — which changes less than it looks. The range printed for the HD 600 comes with no tolerance beside it, and a range without a ±dB figure says nothing about how loud the headphone is at either end of it: thirty decibels down at the bottom and the sentence would still be true. On the only reading that would have made it a specification, the two sheets say the same amount: nothing.
None of this is a measurement. What each maker publishes, and what it is worth, is set out in the frequency response audit.
Each model has a page of its own with the full source list, the hiss estimate and the provenance of every figure above: Sennheiser HD 600 and Sennheiser HD 660S. Every figure on this page comes from what Sennheiser published, or is derived from it — what that maker publishes, counted.