The phrase "subtle refinements" means small and delicate improvements or adjustments made to something to enhance its quality or make it better. Full definition
An audio epiphany, Prime Elevation speaker direct radiating with the charged dynamics, pinpoint accuracy and subtle refinement of a high end bookshelf speaker. (svsound.com)
Subtle refinements in brain architecture, rather than large - scale alterations, make us smarter than other animals (scientificamerican.com)
In fact it's seemingly identical to its predecessor at first glance, save for some very subtle refinements like sharper edges, improved grip with a less slippery back, and relocating the power button to the side to make it easier to reach. (techspot.com)