Is the Universal Audio Volt 476 loud enough for the Sennheiser HD 660S2?
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.
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.