People absolutely can control quality. A simple example is handwriting, I can write chicken scratch or something neater if I slow down. Working longer on many creative pursuits will improve the quality, by experimenting with ideas.
Only up to their quality limit though. This is a slightly different concept to a quantity limit (which also exists), but the general (imperfect) idea is that for your "quality level" (i.e. your ability ceiling), the only real knob you can dial is quantity. In practice, quantity seems to be a defining factor for pushing your ability ceiling higher.
