HD 660S2 is 300 Ω, not 150: every amp recommendation written before 2022 is about a different headphone
104 dB/V at 300 Ω means 1.995 V, 6.65 mA and 13.27 mW for a 110 dB peak. The Volt 476 delivers 2.68 V, reaching 112.6 dB — enough, with 2.6 dB spare and nothing to waste. The current is trivial; the voltage is the whole problem.
Signal Chain Compatibility Matrix
Select gear and parameters for live gain, impedance and SPL calculations.
Input Equipment Setup
PASS: Optimal Signal Chain
Requires minimum +60dB of clean pre-amp gain. Most budget entry interfaces max out at +50dB to +56dB and introduce hiss at max pot. An inline preamp booster (Cloudlifter CL-1 / FetHead) is strongly recommended.
The interface preamp provides sufficient headroom.
$538
$405
-25%Tonal & acoustic characteristics: Flat, wide-range frequency response for clean music and speech. Smooth, warm acoustic character with air-suspension shock isolation.
Technical advisory: With +69dB max gain, the Gen 4 Solo can drive demanding dynamics like the Shure SM7B natively without requiring an external inline booster.
Physical Interconnect Topology
Shure SM7B
Direct Cable Connection (No Intermediate Box)
1x Shielded Balanced XLR Cable (3-Pin XLR Male to XLR Female)
Focusrite Scarlett Solo (Gen 4)
Host: Apple MacBook Pro (M1/M2/M3/M4)
The HD 660S2 went back to 300 Ω after the 150 Ω model whose name it almost shares. One digit in the name, a factor of two in the load.
Sennheiser rates the S2 at 104 dB (1 kHz, 1 Vrms) — per volt, stated outright, no conversion needed. A 110 dB peak therefore needs 1.995 V, 6.65 mA and 13.27 mW. Universal Audio publishes 12 mW into 600 Ω for the Volt 476 headphone output, which is 2.68 V, so the chain reaches 112.6 dB SPL: 2.6 dB of headroom. It plays, and it plays without drama, but 2.6 dB is not spare capacity — it is the difference between a loud master and a quiet one.
Why the older advice does not transfer. We do not have the 150 Ω predecessor in this catalogue, so we will not quote its sensitivity. What the impedance change alone guarantees is this: at any given voltage, a 150 Ω load draws exactly twice the current of a 300 Ω one. Advice from before 2022 was therefore written about an amplifier being asked for a different thing — more current, less voltage per decibel. When someone says "this drives the 660S fine", the sentence contains a model number that has since changed underneath it.
The pearl: 300 Ω is the easy load for current and the hard one for voltage. 6.65 mA is nothing — a phone output can source that. 1.995 V is more than most dongles reach. That is the entire reason 300 Ω headphones acquired the reputation of being hard to drive, and it is why "power" is the wrong word for the question: what runs out is volts. Notice that the required power, 13.27 mW, is barely different from what a 38 Ω headphone of similar efficiency asks for.
What we cannot check, and one thing that fits. UA publishes exactly one operating point for this output — 12 mW into 600 Ω. Whether the same 2.68 V still stands into 300 Ω (which would be 23.9 mW) is not published, so 112.6 dB is the optimistic reading. No output impedance is published either; the 1/8 rule wants below 37.5 Ω here, and 300 Ω is forgiving enough that most outputs would pass — but forgiving is not the same as verified. On the mechanical side this pairing is unusually clean: the HD 660S2 ships with a 6.35 mm cable and the Volt's headphone jack is 1/4", so nothing needs an adapter.