Okay i know nothing about your car and its set up.

But what i would say is no head unit i know of gives out 12V on its low level outputs only the speaker drives would do that.

But consider this each stage of amplication will add more noise to it so amping up to 12V from the empeg will add more noise before adding it to your excisting amp.

I have no idea to the output of a normal amp but it might be cheaper and easier to just put in an normal amp to replace the bose amp.

Another factor is some folk have an Bose setup in audi TTs and they plug straight into the amp with no preamplification so it might work without an amp.

Another thing to mention is the empeg confuses folk due to it being an MP3 player if you say it is a high end head nit without speaker drives/internal amp you may get different answers.