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I present to you the fourth hit in Google for "this den of ": http://www.memphisdailynews.com/news/2012/nov/15/this-den-of-grizzlies-players-doesnt-bluff/ "This Den of Grizzlies Players Doesn’t Bluff"

They were three words that were perfect for summing up Zach Randolph. Three words that were perfect for describing the Memphis Grizzlies seven games into this NBA season...Former Grizzly Shane Battier had the task of trying to guard Randolph. “I’m a suburban cowboy, didn’t grow up on a farm,” Battier said. “But the closest analogy I can think of is trying to wrestle with a steer. Zach is a strong dude.” That was evident again at OKC as Randolph notched his seventh double-double in as many games (20 points and 11 rebounds before he and Perkins were ejected for their verbal sparring); the Grizzlies won their sixth straight to improve to 6-1 on the season...The morning after the Grizzlies had defeated the Thunder, they even worked their way into the conversation on ESPN’s “First Take” with Stephen A. Smith and Skip Bayless. Smith called the Grizz “rough and rugged.” He also said, “They don’t try to be pretty. They know who they are … .”...As the Grizzlies were putting away the Heat at FedExForum, the huge video board focused on fans in Heat jerseys leaving early. “I don’t think people are gonna stop being Heat fans,” Gay conceded. “But I think we gained a couple of more fans tonight. We’re kind of under the radar, but teams in the NBA definitely respect us. They don’t take us lightly and I think we kinda enjoy being under the radar. As long as people respect us, that’s all that matters.”

Obviously positive connotations, but if you somehow wanted to dispute that, on the second page I get two Google Books hits for dens of rattlesnakes which is a neutral objective description.