I've been shooting my 6D for 6yr now and haven't felt the need to upgrade. I shoot event, concert, and bands at clubs mainly without flash and love the low light capability that it has. That being said I shoot in mainly manual modes and have no need for very many bells and whistles.
With a budget of 500-600 dollars I would suggest checking used cameras with lenses from B&H, KEH, or Cameta. Personally, I have bought many used items from Cameta and have been 100% satisfied.
I was shooting a T2i mostly biker rallies and rock groups. I bought a 6D for the low light capabilities and it made a big difference in my abilities. Bought a 24mm pancake lens and a wrist strap, put it on the T2i, now I have a compact street camera.