FiiO K11 with the Sennheiser HD 560S: the problem is too much, not too little
12.5 dB above what a 110 dB peak needs, which is a factor of 4.2 in voltage: the whole listening range lives in the first quarter of the volume knob. On the same DAC the HD 600 sits at −0.5 dB. Thirteen decibels apart, and only one of the two questions ever gets asked.
Yes, the K11 drives the HD 560S. It drives it 12.5 dB harder than necessary, and that is its own kind of problem.
Sennheiser publishes the HD 560S at 110 dB/V and 120 Ω. A 110 dB peak therefore needs exactly 1.000 V — 8.33 mW, 8.33 mA. The K11 delivers 4.223 V after its 1.2 Ω output, so it reaches 122.5 dB with 12.5 dB of headroom. Twelve and a half decibels is a voltage ratio of 4.2: everything you will ever listen to happens in roughly the first quarter of the volume control's travel, where channel-matching errors on an analogue potentiometer are largest and where any noise floor the DAC has is closest to the music.
Compare the same headphone on smaller sources, and the shape of the answer changes completely:
| Source | Output | Level | Headroom | Ratio |
|---|---|---|---|---|
| FiiO K11 | 4.24 V, 1.2 Ω | 122.5 dB | +12.5 dB | 100 |
| Focusrite Vocaster One | 1.64 V, 5 Ω | 113.9 dB | +3.9 dB | 24 |
| Focusrite Scarlett 2i2 4th Gen | 2.45 V, 50 Ω | 114.8 dB | +4.8 dB | 2.4 |
| FiiO KA13 dongle | 1.22 V, 1.7 Ω | 111.6 dB | +1.6 dB | 70.6 |
The €25 dongle is the closest fit on this table. The interface with the biggest voltage is not the best match, and the interface with the second-biggest — the 2i2 — is the only one whose impedance ratio drops below the classic 8.
The pearl: 110 dB/V means 110 dB from one volt, and that is the cleanest number in headphone specifications. A dB/V sensitivity needs no impedance, no conversion and no assumption: the required voltage for any target level is 10^((target − sensitivity)/20), and here it lands on 1.000 V exactly. A dB/mW figure cannot do this — it has to be converted through the impedance first, and that conversion is where most of the arithmetic errors on this subject live. When a manufacturer gives you dB/V, believe it and skip the algebra.
On the same K11, incidentally, the HD 600 sits at −0.5 dB. Same DAC, same afternoon, 13 dB between two Sennheisers.