Sorry for bump, but thought I provided the exact values.
With amp set to 0dB, Clip+ outputs 592mVrms. For most uses, it's safe to take it up to +3dB - I couldn't hear any distortion with 1kHz sine. That's 814mV
+6dB equals 1012mV, with some highest peaks clipped. Noticeable on 1kHz sine, not so much on normalized music content.
So it appears Clip+ even with highly constrained dimensions and battery voltage, manages to hit impressive 1Vrms into unloaded output.
Fiio M3 comes out slightly better, at full volume, it hits 990mVrms undistorted with slightly better battery life. Though the drawbacks compared to Clip+ are enough for anyone to make informed decision in favour of Clip+
Both DAPs are nowhere near X3II or X5II, boasting at 2.67Vrms and 2.82Vrms respectively. Though compared to Clip+ those Fiios are total powerhogs, taking up to 15 times more power than 80mW-ish Clip+.
Though the absolute portable winner into 600Ohms is O2 at 7.5Vrms as far as voltage is concerned.