Best Pianoteq sound quality in a church

Any advice about the mic setup and other Pianoteq settings?

Definitely try totally dry first.  However, you may want a very light reverb (small room) to give it a (very, very light) touch of warmth or presence.  When working with orchestras from samples or individual recordings being merged into an ensemble, you generally want two layers of reverb to simulate the difference between the "stage" sound within the hall and the "hall" sound itself.

For mic position, I agree to start with mono and see what it does.  Keep your signal chain as simple as possible and only "fix" major issues.  Being that you're in such a good space, give the space as much chance to do you favors as possible.  If you go with custom mic placements in PTQ, I'd go with a simple microphone on axis with the lid open for starters and experiment from there.  You want there be enough space for the virtual microphones to even out of the piano sound, as being too present (and close to the hammers) will likely mess with the realism you're going for with classical (e.g. not an issue with pop vocals).

If you're using multiple Bose speakers, then I'd recommend you place them in the room in the position that the virtual microphones are in PTQ.  If only using one, that's less of an issue but still a consideration (it depends on how much that particular Bose unit that I've never used myself adds "space" or "width" to the sound image of its input--trying it out if you can will be far more helpful than any of my theoretical ramblings).

Very good advice, thank you all. Hopefully we can find good testing places before the performance. Only a couple of hours available in the church where the concert will be.

I have recently been to a church where the sound was too loud. I was sitting directly in front of one of the speakers and I had to move because of the distraction it caused. The music was vocal with a few supporting instruments. This was an old "cathedral" type of church meant for a pipe organ which always sounded very good.

Let's not forget the real physic world.

The place where speakers will be positioned and the direction they point is very important.

Is the kind of church where people sing along?

I just wanted to express my personal experience, but to answer you, there is no way I can know this for certain. I expect a piano might be ok because the direct sound would be much louder than the reflections and have a single source. I also believe it would depend on the type of music played and its tempo.

Certainly loudspeaker is more annoying if too close. Real instrument can also be annoying, but it should not be a surprise.