But regardless of the choice here, I just get the default ACE reasonable synth sound. I tried with Calf organ, and got the same result - it sounds like an organ whatever I choose from the dropdown list.
I’d love to try to play a Steinway or some of the other instruments. How do I achieve that?
The button you are clicking is to change the MIDI device (hardware) you’re using. Look at this page in the manual : The Ardour Manual.
To change the instrument, you should click on the plugin itself, the red button in the mixer slice “ACE Reasonable Synth”.