I wanted to mention that Pianoteq does provide a linux VST which does work in Ardour and Qtractor. However, the default version provides 5 output channels which Ardour does not seem to like (because it expects stereo output). However, if you append "_2chan" to the plugin library name (e.g. Pianoteq.so to Pianoteq_2chan.so) , the plugin will output only 2 channels and work happily in Ardour.