If they get loud enough on your focusrite and you still don’t like the way they sound, return them.

Many people make the mistake of trying to “fix” headphones they don’t like with DACs and amps. You came from dynamic headphones to planars and it’s possible you just don’t vibe with the planar sound characteristics

Why not use the streaming functionality of the Denon? Using a lossy bluetooth codec to listen to lossless music kind of defeats the purpose. An external DAC won’t improve the sound over the built-in DAC of the Denon IMO.