There's a merengue festival coming in November and that's going to be held in Puerto Plata. Do a search for it on the board and you'll find plenty of details on that. However, if that's not your time frame then I suggest Santo Domingo.
SD is the capital city and there's always something going on, plenty of clubs to catch live bands. I would suggest checking out the schedule at the "JetSet". It's a nice clean club, more mature crowd and best place to catch a live band. Usually gets packed on the weekends.