(Image credit: Roku) Early on in my TV-testing career, Roku TVs were a bit of a tradeoff. They offered a simple way to stream movies and shows via the built-in Roku interface, but most of the time, their so-so hardware left a lot to be desired. Nowadays, Roku TVs are built better. Take the Roku Pro Series, for instance. Unlike many of its Roku-branded predecessors, this set leverages some relatively sophisticated display technology: Mini-LED backlighting. Today, the 55-inch Roku Pro Series is just $599 at Amazon — its lowest price ever. The Pro Series is equipped with both quantum dot-color and Mini-LED backlighting. If you care about securing decent picture quality with the purchase of an affordable TV, these are two enhancements you'll want. Thanks to its Mini-LEDs, the Pro Series' backlight control is better than what you'll see on basic LED TVs (especially edge-lit models). Better backlight control spells better contrast, and contrast is a cornerstone of picture quality.…