While visiting the Day of the Opened Doors in the Opera from Vienna, I also visited the chorus room. You can see it in the picture.
As far as I realized, the room is very old (just like the building, actually), it has enough place for a huge number of the chorus group and it also has a very good acoustic. Another great thing about this room is that it is placed in the last floor of the building. That happens because an opera chorus can be very loud and the Administration didnâ€™t want to disturb the people from the street or surroundings. Well thought, I must admit!
Unfortunately, there was no one singing there. I would have appreciated to hear someone live.