The scale of DS2 has been retconned and now it's currently set at the ridiculous scale of 900km. Previously the design for DS was made bigger to overcome the original weakness in the original battle station (120km) and the exhaust port was replaced by several micro ports along the super structure leading to the surface... Therefore, on a larger surface area of it's main reactor two proton torpedoes would not be enough to cause the chain reaction to destroy the station. Had DS2 had been completed, it would of been unstoppable. The original size for DS2 was 160km before insanely odd retconning... (I mean it takes over 2 decades for the first DS to be fully operational, and then to build something more than 4x bigger takes less than 3 years? wtf...? 160km makes a lil more sense than 900km due to the logistics of it all I'd think...)

Also on DS2 there are 9 beams as the middle of the dish also produces a beam that converges on the focal point before firing (the orginal has 8 beams actually that converge supposedly but in the movie you can only see 7). As for the exact location of the beams/tubes aka superlaser elements... Each super laser element draws directly from the hypermatter reactor prior to firing. The 64 tubes are there on the original Death Star because apparently there are 8 tubes focusing into 1 of 8 converging rooms... Then from these 8 rooms the beam is channeled into the super laser element that converge again to create the planet cracking super laser. Thus your total of 64. =D

http://www.youtube.com/watch?v=j8TwJBNsbEI&feature=related

Check timestamp 6:40 as to where the location of the Emperor's throne room is on the first Death Star. As for the throne room local on DS2... it's totally ambiguous but if it's like the first Death Star it maybe located around the super laser dish area.