If I had to do a very brief comparison, signature wise, the ha300 is warmer, sweeter, more relaxed, and more spacious and grand sounding, whereas the mz3 is a cleaner and more neutral approach with a greater focus on depth and placement accuracy than width or grandness, speed and clarity, overall more forward, more macrodynamic vs the slight micro focus on the ha300. Generally the mz3 feels much more reference and a focus on accuracy where the ha300 is more focusing on a bit sweeter and more fun approach. I’d say both compete on a similar technical level and trade blows. Headphone wise, the ha300 is less picky from a synergy perspective than the mz3 and is more versatile, but for the headphones and dacs that do pair well with the mz3 they pair really well. From a speaker perspective, you are also more limited on what you can drive with the mz3 whereas the ha300 handles a wider range of speakers better, but I honestly wouldn’t buy either if speakers are your main focus
Also check out the LTA thread which has some good info imo