I listen to a lot of my music through my phone on a set of wireless Sony headphones. Grand for most other stuff but I find metal can sound a bit, flat, for want of a better description. I'm looking for some advice on a good EQ app and what frequencies in it that I should really be focusing on in order to get the most out of metal songs. Any thoughts?

What model number of Sony headphones do you have? Sony have their own EQ app. Not sure what model numbers it's compatible with.

https://www.sony.ie/electronics/headphones-connect-app

I've always been of the mindset to leave EQ and other DSP alone and listen to everything "how it's meant to be". It's going to be hard to find an EQ preset which works for everything rather than just one album. If you're trying to make up for weaknesses with the headphones then you'll just need to find the problematic frequencies and boost/cut them to your liking.

I've heard good things about Viper (assuming you're on android)