I heard the powered ATC monitors at the last Rocky Mountain Audio Festival, and they were some of the best speakers at the show. The ATC midrange driver, in particular, is a legend in the speaker industry, and ATC did a really good job with the active crossover and the internal power amps.
The Raven can easily power 20 to 50 feet of balanced cable with its 4.5:1 step-down output transformer, so it should be an excellent match for the ATC monitors. Studio-grade Mogami or Belden XLR cables should be quite good, but there’s Cardas, Dueland, or Anticables if you want to spend more. I would stay away from high-capacitance cables that look like garden hoses. Capacitance is not your friend when it comes to long cable runs.