Hi Toril,
The Main Line Outs on a Tyros (all models) are controlled by the Master Volume control.
However the Aux Outs are not, and are at a fixed level.
Previous Tyros models had a separate dedicated socket for the TRS speaker system, which meant that you could still use the Main Line Outs for recording, or a mixer, or another speaker system. However Yamaha changed the connection for it's TRS-MS05 speaker system on the Tyros5 so that it used the Main Line Outs.
I assume that you are using the Aux Outs to feed the Focusrite interface and the signal from the T5 Aux Outs are likely to be overloading the inputs on the interface and distorting.
I don't know which Focusrite model you have, but on most audio interfaces there are Gain (or level) controls on the inputs.
So you should be able to adjust these (presumably lowering them in your case) to reduce the signal level.
There is also usually a switch on the inputs to select for use with the very low levels from a guitar or microphone, or the higher levels (line levels) from a keyboard or similar device. So you should also check for such a switch and set it to the appropriate setting. You may well have been able to use the S970 on the more sensitive setting since you can turn down the Master Volume control on the keyboard to reduce the level of the signal going to the Focusrite.
Regards
Ian