AKG - K340 Electrostatic-Dynamic Headphones

Hi all. My lovely wife asked me what I wanted for Christmas, and I was thinking of a nice little headphone music listening station where I usually sit. The amp is covered - Schiit Jotunheim 1 with a new DAC card. My thing is headphones - there are so, so many. I'd like a good reproduction of the music I listen to, which is most everything - classical, jazz, rock, electronic, you name it. From Jefferson Airplane to Earth, Wind, and Fire, to Gordon Lightfoot, Dean Martin, Yo-Yo Ma, Beethoven's "Moonlight" Sonata, Tron: Legacy... you get it. My current headphones I enjoy are an old pair of AKG K340, the "balanced" kind, and a pair of Audeze M-100. They sound nice and are extremely comfortable. I also have a pair of IEM's, B & O H3. Also very nice. What would sound different? What would enhance my experience? What will last years and not cost the world? Budget is $500. I've heard amazing things about Focal, Meze, and of course Sennheiser. Hifiman I've heard many tales that they're very good... briefly. I deeply appreciate the help!