I think the problem is on the monsoon amp, which it is expecting a 12V speaker level input.. I think I have no hope connecting the empeg line level output to the amp directly...

What I'm trying to say is that you shouldn't be trying to plug the line levels into a factory amp that wants something other than line level. You shouldn't have tried plugging into the amp directly in the first place. They make interfaces for this sort of thing. This is covered in the FAQ. You should be looking for an interface to the monsoon system.

I don't know if the PAC OEM-1 or OEM-2 will do the job for you, but I'm sure there's some kind of an interface available somewhere.