@joshua I’m 99.999% confident the polarity is good. All LEDs were oriented the same way and I double checked the position of the long leads for them all. Whereas my placement of the SMD resistors was particularly dodgy! That said, the resistor was probably only for managing the voltage across the LED itself, and there was nothing coming out of the jack.

The good thing about building the kit though is that you get all the schematics so it’s easy to narrow it down.