* Shares in Stada up more than 13 pct (Adds further details, background) : FRANKFURT (Reuters) - Private equity groups Bain Capital and Cinven secured control of German generic drugmaker Stada (STAGn.DE) with a sweetened takeover offer in the largest acquisition of a listed German company by buyout groups, ending months of uncertainty. Investors tendered more shares than the 63 percent minimum acceptance threshold on which the bid was contingent by the Wednesday deadline, Bain and Cinven said in a statement on Friday which was confirmed in a separate release by Stada.