sfdude's system

[David A]

I'd want to try them both out with a quick setup, if you are lucky they both will work well, and you can chose freely.

The similar dimensions of the bonus room, with the height being half the other dimensions might cause big problems with room modes, but I've had an old system in a room that was almost a concrete cube and it work amazingly, for reasons I never understood.

Actually on the dimension side, 19' is very close to 20' and 10' is half of one dimension and a fraction over half of the other. The room is almost half a cube.

Room dimensions aren't the whole story, however. Yes, they determine what the modal frequencies will be but where the speakers are placed determines how strongly the modes are excited and where you sit determines how strong the audible result of the mode will sound so you have 2 modifiers, speaker position and listening position, that make a big difference to how the room modes will affect what you hear.

There's another trick. When it comes to the side to side room axis, the near side wall reflections from the 2 speakers cancel out if the speakers are symmetrically placed, strongly weakening that axial mode. Placing the speakers at the quarter and three quarter points across the room effectively cancels the side to side room mode. Speaker placement will play a big part.

I think @sfdude's big problem when it comes to choosing a room is that 6' grand piano. Really the downstairs room is almost certainly the only practical choice for where to put it because getting it up the stairs and into the room upstairs would be a nightmare if it is at all possible. If you're going to put it in the downstairs room, where you put it severely limits your choices for speaker setup and there's the problem of seating. About the only place you could put it where the seating arrangement is going to work for both listening to the speakers and listening to the piano is right between the speakers and that is not a good location when it comes to speaker setup. If you put the piano on one side of the room and the speakers on the other, then you need to rotate the seating 180 degrees depending on what you people are listening to and if you put the piano on one side of the room and the speakers at an adjacent side, then you're rotating seating 90 degrees  depending on what you listen to. In addition a piano needs some space around it and a 6' piano is going to demand half the room or more in space if it's going to sound good. A system can share a room with a piano but I'd prefer a bigger room if I were going to want to have both the piano and the system performing well.