Optimal Headphone Gain for WiiM Ultra with Sennheiser HD650?

[Fralle]

Hey everyone,

Following up on my previous discussion regarding the optimal line-out level for my Audiophonics power amp (thanks for all the great input there!), I’ve now turned my attention to the headphone output.

I primarily use a pair of Sennheiser HD650s.
Currently, I have the output level set to 500 mVrms, mirroring my speaker setup for consistency and safety.

Once again, I’m seeing conflicting advice:

1. One camp suggests keeping the level low (500 mV - 1 V) to keep the noise floor as low as possible.
2. The other camp (and my AI collaborator) suggests cranking it to 2 Vrms to give the high-impedance Sennheisers the voltage they need to control the drivers properly, especially when using EQ with negative preamp/auto-gain.

For those of you using high-impedance headphones like the HD600/650 series with the WiiM Ultra:

• What output level are you running?
• Have you noticed a difference in "drive" or bass authority when moving from 1V to 2V?
• Does the higher gain introduce any audible hiss or noise during quiet passages?

I’m curious if the "headroom" logic we discussed for the line-out level applies differently here when the WiiM is acting as the actual headphone amplifier.

Looking forward to your thoughts!