Sennheiser HD650 Upgrade

[radix]

Is application based PEQ only usable when playing "local" files, i.e. will it work on streaming sources like Apple Music, Spotify, etc?

As long as the sound goes through your computer, you can application EQ it.

I use a Mac and SoundSource, which has a bunch of headphone EQs pre-bundled. If I use Spotify on the Mac, it goes through SoundSource to the headphones. SoundSource will EQ any output devices, even airplay. If I use Roon, I do the PEQ inside roon and skip routing through the computer.