In your opinion, what is a good instrument?

A good instrument sounds beautiful, is a pleasure to play and looks good.

Let's take this point further; when can you say an instrument sounds beautiful?

That's already rather more complicated. Just as every person has their own voice, so does every guitar. Whether or not you find that sound beautiful depends on your personal taste, on your memories and associations. Some guitars have a round sweet sound, and there are others with a clear or with a sultry, hoarse tone. Each has its own character.
In making an instrument, I strive to let the guitar speak freely and in a relaxed manner. In addition I want a broad palette of sounds; the extremes of sound production must be within reach.

How do you achieve that?

All the techniques and materials together determine the final result. The type of wood and its thickness, the model, the bars, the varnish and so on, all really play a part. But I can only hear how it sounds in the final stages when I mount the strings.

Apart from adjusting the tuning, is it possible to influence the sound production after putting on the strings?

Oh sure! It's only in the sounding phase that I begin to discover the personality of the instrument and to bring this out to its fullest potential - for example through experimenting with the position of the bridge, strings and soundpost.