I think an upgrade from HD650 for $500 is hard to pull off with headphones. The best upgrade paths are electronics - for maybe $150 a Quedlix 5k and “balanced” cable, for around your budget you can add a miniDSP Flex if you have separate DAC and amp, or for over your budget an RME ADI-2. The reason is then you can play with bass shelves (this headphone doesn’t really need anything else), crossfeed, and (for RME) loudness in the included DSP.