Sony recommendation is 200 hours burn in for each output. General consensus is that the 4.4 balanced is the only way to go on the zx300.
Mine was purchased used with around 50 hours on the balanced side. At that point, a few songs sounded good, a few just ok, and some like absolute crap! Intolerably bad. I hit high gain, play all, plugged it in, and ignored it for a week. Phenomenal change! All the songs that sounded like crap sound good. Really good!
Of course now I’m wondering if upgrading from the Campfire Audio Lyra II’s could take it up another notch........
I’ve pretty much settled on low gain, and direct sound for most listening, at least with high quality recordings. Preserves the most detail. The upsampling and vinyl processing are fun to play with, and can certainly improve recordings that are poor to average, or odd re-mixes. I typically use type A low, Strings, and vinyl processor on standard.