It is actually easiest to get to those rooms through the basement of KTH. Really hard to explain where it is, but if you get to KTH B135 and then head down the hallway facing directly away from the doors to get into KTH B135 you should see a hallway. Go down this hallway until you get to your + junction and take a right. At the end of this new hallway is just a regular looking door, open it. Step into the realm that is the AB basement of DSB
And just so you know, KTH B135 is the only "lecture hall" in the basement of KTH. It is the one that is right to the right after walking down the stairs in KTH