This is what I posted earlier:
"If you are plugging left and right line-level output into the input of active speakers, you will get a good clear sound with stereo separation.
"If you go via the interface, it is highly likely that, even if you plug into two inputs on the interface, you are actually only getting a mono sound from the two speakers, unless there is a facility on the interface to pan the inputs to left or right. Most interfaces don't have this, though it can often be done with software."
Did you investigate that possibility? One quick way to test is to connect keyboard to interface, record a bit to a stereo a track (or to two mono tracks and pan them left and right), then playback.
Thank you all
Problem is Solved
Hurray
What I have done:
My Soundcard is Apogee Element 46.There is a snapshot template at the software of the soundcard what gives direct monitoring.
So I can hear the sound without daw.
In this direct monitoring I see four channels, corresponding to the four xlr inputs at the front of the sound card.
Above each channel in the software menu is written analog. I assume each channel is a mono analog signal.
For each channel (1-4) I can manage the gain level. And I can group the channels together and make from two channels one stereo channel.
So My piano has two xlr outputs what is named balanced out. left and right. They go to the xlr input at the front of the sound card corresponding to channel 1 and 2 in the software menu.
I have grouped channel 1 and 2 together and pressed the stereo button in the mixer 1 menu of the software of the sound card to make one stereo channel of channel 1 and 2.
In combination with enough gain, but not too much I get a sound what is almost the same as the original piano sound.
I am very happy
.
I still love to know why. How come is there such a (little) difference in the sound between this settings? My piano sends a mono sound to channel 1 and a mono sound to channel 2 right?
When I play then on my piano I hear sound from both speakers left and right. Then I group channel 1-2 and make them stereo. Panning is still in the middle, set to zero, by the way. And the sound changes..... How does this happen?