Phoenix Gold used to have something called a Line Driver, I think it was PLD-1 for a model number. I had one in my last car. I'm sure they don't make them anymore, but you might find one on ebay? It had a single input and 2 or 3 outputs that would go as high as 8V, and I don't remember it adding any noticeable distortion or anything.

*****edit*****

Wow, crappy memory. One input, one output. It still may be what you need for that channel. There's at least one on ebay right now: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3041266847&category=14933 . I think they were about $100 when new.