Can I use a split?

While the DAC has a tube-based output stage and therefore might have a relatively high output impedance

For sure, it only has 1 dual triode, half for each channel. If it had two then it’s possible it could have had a white cathode follower output which "can" be quite low, I have a Elekit preamp here which has CF output it and it’s around 100ohms output full range, using a huge coupling cap naturally.

Cheers George

Tell us the model of the dac and it's "output impedance"
And the "input impedance" of the amp (model), and headphone amp (model).
And we’ll work it output.

Cheers George