Thank you for the great review! Can you compare it with the FiiO BTR7? What is your opinion?

Thank you for reading!
As expected the extra amplification stage and the better power supply result in an improved overall sound quality.
Fuller, more dynamic, wider, a touch more balanced and refined etc.
Still the Q5 is less expensive and is the tweakers delight and has the best EQ and the Chrome extension.

Thank you, I own and use both
Have you reviewed the BTR7?
Also, according to your opinion, which one could, theoretically, drive the Meze 109 Pro better? The 5K or the BTR7?

Yes.
https://www.head-fi.org/showcase/fiio-btr7.25949/review/29532/
The BTR7 by a fair margin.