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.