This thread provides a great question and very helpful answers.  I have always wanted my Bluethner to be a bit more bright.    I have always messed around with the hammer hardness, mostly because that is obvious.  It is nice to know what needs to be done with the other parameters.

You could try:
Adjusting the [Aliquot Strings] in the [Design] section. To the right creates a darker tone, while to the left a brighter tone.
In the [Output] section: [Effects]; [Reverb] adjust the [Tone] slider, to the right creates a darker tone, while to the left a brighter tone.

1. Increase the unison width ( between 1.10 and 1.20);
2. Decrease hammer hardness (each -0.05);
3. Increase hammer noise a little bit;
4. Increase first overtone in the spectrum profile (1 db);
5. Close the lid (not completely) to boost the bass and to soften upper range.

See if this works for you.

This thread provides a great question and very helpful answers.  I have always wanted my Bluethner to be a bit more bright.    I have always messed around with the hammer hardness, mostly because that is obvious.  It is nice to know what needs to be done with the other parameters.  What I did was the reverse:
1. Decrease unison width to 0.9
2. Increase hammer hardness (all three) by +0.05
3. Decrease hammer noise
4. Decrease the first overtone in spectrum to -1  (this is the one I questioned, I assume for brighter I don' need to touch the other overtones)

On first take it does seem to do the trick.  Thanks guys!

His directions to you were how to make any piano sound "darker" (less bright) than its original preset.

Making hammers "softer" (i.e., less hardness) gives the piano less high overtones; the opposite is also true.

I can't describe this in more concrete way, but I think everybody feels that Bluethner has more dark and heavy sound, and D4 has more light sound. But how to adjust sound in this direction: make it more dark or more light? Which Pianoteq's setting should I touch for that, with minimizing changes of sound in others directions?

1. Increase the unison width ( between 1.10 and 1.20);
2. Decrease hammer hardness (each -0.05);
3. Increase hammer noise a little bit;
4. Increase first overtone in the spectrum profile (1 db);
5. Close the lid (not completely) to boost the bass and to soften upper range.

See if this works for you.