Sennheiser HD 600 on a Scarlett Solo 4th Gen: loud enough?
The Solo reaches 2.45 V, the HD 600 asks for 4.47 V at a 110 dB peak. It stops 5.2 dB short — and swapping the interface for a louder one does not close the gap.
No, if you want 110 dB peaks: the Solo stops 5.2 dB short.
The HD 600 is specified at 97 dB/V into 300 Ω, so 110 dB needs V = 10^((110 − 97)/20) = 4.47 V — 66.5 mW and 14.9 mA. The Solo's headphone output is published at +10 dBu, that is 2.45 V, which reaches 104.8 dB.
That is not silence. 104.8 dB of peak means comfortable listening at roughly 80 dB average with the 25 dB crest factor of an uncompressed recording, and modern loud masters will sound loud. It runs out precisely where the HD 600 is worth owning: material with real dynamic range.
Do not try to fix this with another interface. The highest published headphone output among the interfaces here is the MOTU M2 at 3.27 V, and even that lands at 107.3 dB — still short. The missing 5.2 dB is 3.3 times the power; only a source that swings more than 4.5 V into 300 Ω closes it, which in practice means a dedicated headphone amplifier.
And the honest counter-answer: if what you play is speech, podcasts and pop, the Solo is fine, and the amplifier buys you nothing you will actually hear.