I just got a U2 mini and love it. I had a high end digital coax cable, a mogami digital coax cable and a mogami digital AES cable (xlr). I could not hear any difference between the cables. I think it is because the Lumin and the Dac are so good.
I heard a huge difference between my Esoteric dac and my reimyo dac. I would focus on upgrading your dac. If you have questions on cables Mike Kay of Audio Archon is an expert in digital music streaming plus he is honest and has stopped me from spending more money on cables that won't make a diff.
Best JH