Guitar Center Inc. has agreed to be acquired by private equity firm Bain Capital Partners LLC. Bain will pay$63 per share or about $2.1 billion for the Westlake Village based operator of musical instruments stores and Web sites, a 26 percent premium over the price of Guitar Center shares as of June 26. Guitar Center, through Goldman, Sachs & Co., conducted an auction for the company, Guitar Center said, although the names of any other interested parties were not disclosed. The deal, which is subject to customary approvals, is expected to close in the fourth quarter of this year.