Considering the Nh Deggendorf in Passau?
A VT member wrote the following comment about visiting Passau:
The biggest organ in the world by globetrott
I was lucky when I was inside St.Stephen's ,the cathedral of Passau, as the German Television had a rehearsal for an organ-concert and all of the church was professionally lighted.
The organ has 17388 pipes and 231 registers.
In summer you may attend a daily organ-concert at lunchtime !
For the concerts you have to pay a small entrance-fee, but all the other time, the church is open for visitors without restrictions during the day.