I went through measurements and my findings for the sweet spot with the Khadas Tone2 Pro and a few other DACs in this presentation I did for the San Francisco Audiophile Society:
If that’s too long to watch, I’m using the Hybrid Fast filter. Not only does it sound great, it seems to have the best time-domain response.
For connections, I highly recommend using one of the Khadas balanced RCA cables if your external amplifier has XLR inputs…and especially if it’s fully balanced. I’m getting great sound out of the 4.4mm balanced headphone jack on the Tone2 Pro; however, results are significantly better from my THX AAA 789 headphone amp with a balanced connection.
You’ll generally get better sound from the Tone2 if you use a better digital transport. A general purpose laptop or desktop PC will work, but you may get better sound from a dedicated, purpose-built streamer, like VitOS from Silent Angel.
Finally, make sure that you have a good power source for the Tone2 Pro. Some smaller devices, like smartphones, tablets, and the Raspberry Pi may struggle a little without using a secondary power source.