Legacy mode is a limited-functionality mode that offers maximum compatibility. Depending on your device manufacturer's audio framework implementation, it should work with most music apps that come preinstalled on your phone. Examples of apps it should be compatible with include Tidal, YouTube, Soundcloud, Qobuz, Neutron, and PowerAmp.
Most major music apps such as Spotify, YouTube Music, Shuttle, Phonograph, and Google Play Music will open their own global audio sessions which Wavelet automatically uses, so you won't need legacy mode for these (thus it's turned off by default). In my experience, SiriusXM's Android app also works without needing legacy mode. Below is an example of how the legacy mode works: In Google Play Music (which as I mentioned doesn't need legacy mode as the left screenshot shows), the Reverberation and Limiter features are disabled (right screenshot). "
The XDA article says: " Legacy mode
Legacy mode is a limited-functionality mode that offers maximum compatibility. Depending on your device manufacturer's audio framework implementation, it should work with most music apps that come preinstalled on your phone. Examples of apps it should be compatible with include Tidal, YouTube, Soundcloud, Qobuz, Neutron, and PowerAmp.
Most major music apps such as Spotify, YouTube Music, Shuttle, Phonograph, and Google Play Music will open their own global audio sessions which Wavelet automatically uses, so you won't need legacy mode for these (thus it's turned off by default). In my experience, SiriusXM's Android app also works without needing legacy mode. Below is an example of how the legacy mode works: In Google Play Music (which as I mentioned doesn't need legacy mode as the left screenshot shows), the Reverberation and Limiter features are disabled (right screenshot). "
And it's still a bit unclear to me what need Legacy mode to work? For Tidal, YouTube, Soundcloud, Qobuz, Neutron, and PowerAmp?" source: https://www.xda-developers.com/make-your-headphones-sound-better-automatic-eq-wavelet/
And I have heard that Spotify, Netflix, etc. doesn't allow other apps to edit they audio streams, how Wavelet can do all of this without root access?