Best place to use better xlr cable

As early in the audio chain is most probable… but, like everything in audio… you have to try it.

‘Questions you might ask, that would suggest a different place to start: are all of you components internally balanced? If not that would suggest you should use it between two with balanced topology.

‘But unfortunately, it could sound better elsewhere. But starting in a logical place makes sense. Depending on your equipment, it might not matter… although unlikely.

What is your equipment?

Everything is balanced, Lumin T1, musical fidelity M6 preamp and musical fidelity m6s amp

Try both options, then use the one that sounds better.  Likely not a great difference either way.

typically source to pre, but not always...