I looked at some of the usual places for AUX input adapters (Blitzsafe, rcainput.com) and none of them seem to have any interfaces for Subaru. Your other options are to get a line level converter to convert your speaker-level outputs on your factory stereo to pre-amp output levels. With the resulting outputs you can then either:

1. Plug them into the AUX input on the empeg
2. Get a switch like the Sony XA-39 or the PIE MPSS-4 to automatically switch between the units

Either way you will need a 4 or 5 channel amp to push your speakers.