I think he means hi-level inputs....... so you can go with a speak line in....... say from the rear speakers. You tap into them.
I have a few amps kicking around with high level inputs that could run your sub that I'm looing to sell. PM me if your interested.
Current : 2013 Mini Countryman JCW, 2007 Z4M Roadster
Past: '88 E30 M3, '69 BMW 2002 Ti, 2000 328i, Audi A6 TT, BMW X5 3.0i , Audi A4, Mini Cooper Clubman S 1.8T