Connecting headphone amp to Luxman C900U-M900U

My C900U has no fixed output neither a tape loop.
How shoud I connect a headphone amp (Musical Fidelity X-Can V3) ?
If I use the RCA out (connection with the M900U is balanced), the output level depends on the volume setting of the C900U.
Is there some danger for the X-Can if the level is too high ?
Any help is appreciated !

It would be very unusual if your XCan would be damaged.  Absolute maximum voltages from line level preamps are usually around 15V, far too low to break through the input buffers.  You could overload it, meaning, the signal would be clipped at the maximum input level, but would be very unusual for you to hurt it.
BTW, I notice you have an "External Preamp" mode.  This may be unity gain (i.e. no volume control).