The Dan Clark Audio AEON 2 Closed is 13Ω at 90.0 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.
The Dan Clark Audio AEON 2 Closed needs 8.8 dB more than the Sennheiser HD 660S2 to reach the same loudness — 10 mW against 1.3 mW.
| Specification | Dan Clark Audio AEON 2 Closed | Sennheiser HD 660S2 |
|---|---|---|
| Impedance | 13 Ω | 300 Ω |
| Sensitivity | 90.0 dB/mW | 98.8 dB/mW |
| Sensitivity (voltage) | 108.9 dB/V | 104.0 dB/V |
| Power for 100 dB | 10 mW | 1.3 mW |
| Voltage for 100 dB | 0.36 V | 0.63 V |
| Current for 100 dB | 27.7 mA | 2.1 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 1.62 Ω | 37.50 Ω |
| Sources that reach 100 dB out of 280 | 272 | 280 |
8 of the 280 sources have the power for the Sennheiser HD 660S2 but not for the Dan Clark Audio AEON 2 Closed — Apple USB-C dongle (EU), Zoom AMS-22, Audient EVO 4, 5 more.
25 sources are low enough in output impedance for the Sennheiser HD 660S2 under the one-eighth rule but not for the Dan Clark Audio AEON 2 Closed (FiiO KA13, Zoom AMS-22, Arturia MiniFuse 2, 22 more).
272 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 Dan Clark Audio AEON 2 Closed than 80%. 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 | Dan Clark Audio AEON 2 Closed power | Dan Clark Audio AEON 2 Closed voltage | Sennheiser HD 660S2 power | Sennheiser HD 660S2 voltage |
|---|---|---|---|---|
| 80 | 0.10 mW | 36 mV | 0.01 mW | 63 mV |
| 85 | 0.32 mW | 64 mV | 0.04 mW | 0.11 V |
| 90 | 1.0 mW | 0.11 V | 0.13 mW | 0.20 V |
| 95 | 3.2 mW | 0.20 V | 0.42 mW | 0.35 V |
| 100 | 10 mW | 0.36 V | 1.3 mW | 0.63 V |
| 105 | 32 mW | 0.64 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, Dan Clark Audio AEON 2 Closed | Headroom, Sennheiser HD 660S2 | Output impedance | Noise |
|---|---|---|---|---|---|
| JadeAudio JA11 | $20 | 0.9 dB | 3.9 dB | 0.5 Ω | 1.7 µV |
| FiiO KA11 | $29 | 13.0 dB | 12.2 dB | 0.7 Ω | 1.7 µV |
| FiiO BTR11 | $36 | 5.6 dB | 3.6 dB | 0.5 Ω | 8.0 µV |
Of the 280 sources here, 272 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 200 of them are in the second group.
The cheapest of the 69 is the JadeAudio JA11, at $20. It leaves 0.9 dB of headroom on the Dan Clark Audio AEON 2 Closed and 3.9 dB on the Sennheiser HD 660S2, its output impedance is 0.5 Ω against a one-eighth limit of 1.62 Ω and 37.50 Ω, and its noise figure puts the hiss floor 27 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 |
|---|---|---|---|
| Dan Clark Audio AEON 2 Closed | “Yes, it has one” | not printed | not printed |
| Sennheiser HD 660S2 | 8 – 41,500 Hz | not printed | not printed |
Dan Clark Audio prints “Yes, it has one” for the AEON 2 Closed. Sennheiser prints 8 – 41,500 Hz for the HD 660S2.
Dan Clark Audio AEON 2 Closed: The specification table has a frequency-response row. The value printed in it is the sentence "Yes, it has one".
One of these two sheets prints a range and the other prints a sentence declining to, and the second is the more informative of the two. The range given for the Sennheiser HD 660S2 carries no tolerance, so it says nothing about how loud that model is at either end of it — which is exactly the objection the other sheet is making out loud.
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: Dan Clark Audio AEON 2 Closed and Sennheiser HD 660S2. Every figure on this page comes from what Dan Clark Audio and Sennheiser published, or is derived from it — what Dan Clark Audio publishes and what Sennheiser publishes, both counted.