Which version of the DT’s did you end up buying? The 32 Ohm version?
The DT770 is not known for being the loudest headphone in the world…but personally, I don’t like to listen to music super loud. I’m super paranoid about damaging my hearing. So I might not be the best person to help you out with this.
As a recent S8 owner myself, I have noticed that the amp isn’t quite as good as the one on my old iPhone…but again, I rarely end up listening much past 65 percent or so. Samsung does include a volume limiter and various warnings to try and prevent you from damaging your hearing, and the phone should be able to drive the 32 and 80 ohm versions of the headphones decently in spite of this. .
I’m not sure that there are any apps that will solve this. You’re probably going to need a portable amp if the volume isn’t doing it for you, and it’s tough to find something that pairs decent battery life and portability.
If you don’t mind staying in the analog realm and just using a 3.5mm passthrough cable, I’d recommend the relatively cheap Soundblaster E1. It has a decent battery life, an exceptional mic for taking phone calls, and a little clip so you can attach it to your shirt and not worry about it taking up room in your pocket. Plus, it has a play/pause button for music right on the side of it.
I haven’t found a product better than this myself, but I haven’t reviewed or used very many portable amps…so I’m nowhere close to an expert. Sorry I don’t have any more to recommend. The E1 should allow you to crank them up pretty loud…and if it’s still not loud enough, well then, you’re braver than I!