Massive gets a lot of referrals from this place and for good reason. He should be at the top of your list. He's right, he is not allowed to advertise his own services, but other members can do that for him.
Unfortunately, I'm more budget conscience and maybe I was a little too humble to send my garbage to him; I found a place called Sage Audio. I feel they did a good job with my material for a decent price. There are a lot of fly-by-night mastering studios out there, it's hard to discern the fakes and wannabes from the real deal. Price is usually a good indicator. Sage had a better rate than Massive, but they weren't fly-by-night gutter trash, either.