The Audio-Technica ATH-M50X is 38Ω at 99.0 dB/mW; the Sennheiser HD 660S2 is 300Ω at 98.8 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.
In amplifier terms these two are the same headphone. The gap between what they need for 100 dB is under half a decibel, which is smaller than the step between two clicks of most volume controls.
| Specification | Audio-Technica ATH-M50X | Sennheiser HD 660S2 |
|---|---|---|
| Impedance | 38 Ω | 300 Ω |
| Sensitivity | 99.0 dB/mW | 98.8 dB/mW |
| Sensitivity (voltage) | 113.2 dB/V | 104.0 dB/V |
| Power for 100 dB | 1.3 mW | 1.3 mW |
| Voltage for 100 dB | 0.22 V | 0.63 V |
| Current for 100 dB | 5.8 mA | 2.1 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 4.75 Ω | 37.50 Ω |
| Sources that reach 100 dB out of 280 | 280 | 280 |
Every source in the database that drives one of them drives the other. On power, the choice between these two changes nothing about what amplifier you need.
19 sources are low enough in output impedance for the Sennheiser HD 660S2 under the one-eighth rule but not for the Audio-Technica ATH-M50X (Zoom AMS-22, Arturia MiniFuse 2, Arturia MiniFuse 1, 16 more).
1 source stays below the audible-hiss threshold with the Sennheiser HD 660S2 but crosses it with the Audio-Technica ATH-M50X (Schiit Mjolnir 3).
280 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 Sennheiser HD 660S2 is more demanding than 91% of the 567 non-electrostatic models in this database; the Audio-Technica ATH-M50X than 62%. 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 | Audio-Technica ATH-M50X power | Audio-Technica ATH-M50X voltage | Sennheiser HD 660S2 power | Sennheiser HD 660S2 voltage |
|---|---|---|---|---|
| 80 | 0.01 mW | 22 mV | 0.01 mW | 63 mV |
| 85 | 0.04 mW | 39 mV | 0.04 mW | 0.11 V |
| 90 | 0.13 mW | 69 mV | 0.13 mW | 0.20 V |
| 95 | 0.40 mW | 0.12 V | 0.42 mW | 0.35 V |
| 100 | 1.3 mW | 0.22 V | 1.3 mW | 0.63 V |
| 105 | 4.0 mW | 0.39 V | 4.2 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, Audio-Technica ATH-M50X | Headroom, Sennheiser HD 660S2 | Output impedance | Noise |
|---|---|---|---|---|---|
| JadeAudio JA11 | $20 | 13.0 dB | 3.9 dB | 0.5 Ω | 1.7 µV |
| FiiO KA11 | $29 | 21.3 dB | 12.2 dB | 0.7 Ω | 1.7 µV |
| FiiO BTR11 | $36 | 13.0 dB | 3.6 dB | 0.5 Ω | 8.0 µV |
Of the 280 sources here, 280 reach 100 dB on both. That is the easy axis. 69 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 208 of them are in the second group.
The cheapest of the 69 is the JadeAudio JA11, at $20. It leaves 13.0 dB of headroom on the Audio-Technica ATH-M50X and 3.9 dB on the Sennheiser HD 660S2, its output impedance is 0.5 Ω against a one-eighth limit of 4.75 Ω and 37.50 Ω, and its noise figure puts the hiss floor 22 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 |
|---|---|---|---|
| Audio-Technica ATH-M50X | 15 - 28,000 Hz | not printed | not printed |
| Sennheiser HD 660S2 | 8 – 41,500 Hz | not printed | not printed |
Audio-Technica prints 15 - 28,000 Hz for the ATH-M50X. Sennheiser prints 8 – 41,500 Hz for the HD 660S2.
Neither range has a tolerance printed beside it, and that is the whole of what this section can tell you. A wider range on one sheet is not deeper bass or more treble on that headphone: without a tolerance, neither number says how loud either model is at the ends of its own range, so the difference between the two is a difference between two pieces of marketing copy. A ±dB figure is what turns a range into a specification, and of the 490 models whose maker prints a range, 58 print it.
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: Audio-Technica ATH-M50X and Sennheiser HD 660S2. Every figure on this page comes from what Audio-Technica and Sennheiser published, or is derived from it — what Audio-Technica publishes and what Sennheiser publishes, both counted.