When HBO unveiled its 15-channel digital multiplex last week, Encore Group Chairman John Sie let it be known that imitation is the sincerest form of flattery. "They even used our spelling of 'Pak,'" Sie said. HBO's new plex is called HBO/Max Pak, combining all the iterations of HBO and Cinemax with a few new ones. More than a year ago, Sie combined all the Starz! and Encore derivations to create the 12-channel Super Pak. However, HBO sources say they, too, are flattered by Sie's imitation of a concept that they initiated in 1991. HBO or Cinemax subscribers automatically get the new Max Pak when they go digital.