I bought and sold a zx500. It basically got rid of everything good about the 300 except for the form factor.
The only thing I’m not happy with is the volume limit (I have an EU machine). It doesn’t leave me any headroom on insensitive planar magnetic headphones like Fostex T50’s and Dan Clark Aeon’s. Worse than that, they limit the 4.4mm output to the same power output as the 3.5mm and have removed high/low gain completely in software in these systems