Not to spoil your fun or anything, but the Vali 2 may not be the best match for your 600 ohm Beyers. At that impedance, you're only getting 140 milliwatts of power, and while the DT880's are reasonably sensitive, I wouldn't refer to them as highly sensitive. But if you like the sound you're getting that's really all that matters. Since you're giving the Vali you have to your son (what a Dad!), you might take a look at the Valhalla for yourself. More money, but 2.5x the power into a 600 ohm headphone. For adapters, if you're in the EU you might want to PM fellow HeadFi'er @Deyan . He does some very nice work and may even cost you less (delivered) than buying from China.