USB DAC not liking external speakers

[phaseform]

I have seemed to have fried a DAC by connecting a small speaker and Sennheiser wireless headphones (both powered by isolated supplies) to it (together through a 3.5mm Y splitter)..

Not sure why this would happen?

Just watching this video
EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
Can't see how I have an issue here..??

[Manul]

Well, I dont know, but isolated supplies are not totaly isolated, they have AC leakage through capacitance if output side is not grounded. Easy way to look for such things is with multimeter. You can measure AC voltage between your USB ground and speakers input jack. But even in cases of leakage, the current caused is usually very low, so it is not likely to damage things easy, but who knows. Measure and maybe it will get more clear.