I bought the Sierras Altas and am very disappointed. The included samples are so quiet that it is impossible to work with this plugin. The ad promises a lot, but the plugin is a flop.
The gain in the plugin was set to 100% and the headphone amplifier from the audio interface was also set to 100%. It could only be seen that the Sierras Altas plugin is receiving MIDI data. It would have been nice if you could have heard the sound too. Unfortunately it wasn't and that's why I can't recommend the Sierras Altas plugin to anyone.