Sennheiser HD 490 PRO on a Scarlett 2i2 4th Gen without an amp: exactly enough

Exactly enough: 110.0 dB against a 110 dB target, headroom 0.0 dB.

Level. Sennheiser publishes 105 dB/V at 130 Ω. Through the 2i2's 50 Ω output resistor, 2.45 V arrives as 1.77 V, and the peak level lands on 110.0 dB. At the more usual 105 dB listening peak there are 5 dB in hand. What the chain has to supply is 1.78 V, 13.7 mA and 24.3 mW — the interface publishes 22 mW at 300 Ω, so this is the edge of what it is specified to do.

Tonal balance. 130 Ω against 50 Ω is a ratio of 2.6 where the rule wants 8. For a moving-coil driver that is a real but small shift at the bass impedance peak, not a failure: the measurement most often cited for a comparable case puts it at 0.3–0.5 dB. We print the ratio and the scale rather than the words «not recommended».

The pearl: the 300 mW on the spec sheet is not a request. Sennheiser prints «Power consumption (Amp) 300 mW (100 h, noise IEC 60268)». Passive headphones consume nothing — that figure is IEC 60268-7 power handling, what the driver survives through 100 hours of noise. Read as a demand it says «bring a 300 mW amplifier», and that reading is what sells headphone amps. The actual requirement on this page is 24.3 mW for a 110 dB peak and 7.7 mW for 105 dB — twelve and thirty-nine times less. A row that describes destruction gets read as a row that describes appetite, and the difference between the two readings is the whole purchase.