There´s no Sinn ...... in Berlin and there´s none in Germany. The only place to get your Sinn is at ADs or right from the factory at Frankfurt (no price reduction). The Sinn AD I know is outside of the centre of Berlin:
Juwelier Stöger GmbH i.G,. Berkaer Straße 6, 14199 Berlin.
A place to be - as already mentioned - is Wempe at Friedrichstraße or Kürfürstendamm. At Friedrichstraße Rene Kern is another well known dealer (GO, Anonimo, Rolex, Maurice Lacroix, Cartier, Ebel and many other brands). Down at Quartier 205 (opposit side of Rene Kern) you´ll find a little watch shop with Candino watches (have a look at their divers).
On Kurfürstendamm you´ll find some very distinguished flagship stores of Cartier, Bulgari. Bucherer, a well known swiss watch shop, just opened at Kurfürstendamm.
Close to the Kurfürstendamm you´ll find Linckersdorff, Fasanenstraße 70.
Linckersdorff is specialized in vintage (high-end) watches.
If there´s time enough you may visit
ASKANIA watches.
Another place to be is the KaDeWe (Kaufhaus des Westens) close to sub station Wittenbergplatz (Cartier shop, Omega shop, Bulgari shop) and a juwelers shop selling GO, Longines, Zenith and other hingh end watches).
Hope this will help a bit.