I've gone back and had a good look at how it is all wired. The OEM Philips headunit that comes with the car is connected via its own Subaru-ISO adapter.

I have checked the wiring on that adapter with the wiring on the new adapter that I am using. The wiring for the memory and ignition lines are the same on both.

So three questions:

- is it really likely that the same wires that could supply enough current for the Philips headunit (complete with four channel amp) don't have enough to supply the empeg
- why do both memory and ignition both have a voltage when the ignition is off (the Philips headunit does not operate when the ignition is off), surely that is going to cause me problems even when I sort out the current draw issue
- which line does the empeg draw its power from when it is awake and running

I guess what I really need to do is run a line temporarily from my amp for the memory line, to see if that works any better. The other thing I will do is to try connecting the empeg via the original Subaru-ISO adapter, just in case there is something subtle and non-obvious wrong with the new adapter.