I haven't been to Bamberg but have read good things about it.
I enjoyed Erfurt a lot in July 2015 (though it was hot, like everywhere else). It survived the war well, and it's a college town so it has that vibrancy in addition to its handsome historic district. Really a very attractive city.
Dresden has fabulous museums; I especially loved the Historic Green Vault. But it was firebombed during WW II. A small part of the city core was restored, but I'm not sure it's the best place for "old world German charm". On the other hand, if you're unlucky with the weather, you might be glad to have museums to visit. On the third hand, you're headed for Berlin, aka Museum Central, so I guess you don't need museums on the way.
One of my two favorite towns/small cities in Germany from the standpoint of aesthetics is Quedlinburg, which is not very far off the Munich-Berlin route. It has over a thousand half-timbered buildings, a castle, and a cathedral with a very impressive treasury (Google for US connection). There's also the Feininger art gallery. I'm guessing the English language walking tours probably don't run in January, but the tourist office has an audio guide to rent. It takes a long time to cover the city, so two nights would be very well spent in Quedlinburg.