If your sources have multiple outputs, such as XLR and RCA, then use 1 output to the preamp and another to the headphone amp. I do this with my Schitt Ygg+ DACs and my Benchmark DAC3B.
I also do this with my Magnum Dynalab MD108T tuner.
Some gear does not support having 2 outputs connected at the same time so check with the company that created the source unit.