I've got a similar enlarger, a C7700 with colour head. My normal printing size from 6x6 is 7" square on 8" square paper, so very close to what you're trying to do. With an 80mm lens, I get typically 20-30 second exposures at f/11, so I'm surprised you're seeing so much more exposure is required.
One thing that occurs to me: How fresh is your paper, and where did you get it from? I've been given some old paper in the past which has fogged so badly that also any exposure will produce a really murky dark image. In fact, a couple of packs were so bad that even the unexposed areas went almost black!