I only now just started reading on the PID for the bed. I have done the autotune on the hotend. It was my assumption that you could not use the PID loop when using the heat bed relay. Is this the case? A quick google of Heat Bed PID made it sound possible. My assumption is that they are not using a relay however.

Also, I have read that document many times and I get something new from it every time. I think I have convinced myself even more to look into a 6mm lead screw with a single start. Can't find them in a 12" maker farm length however. 320mm is about the longest I have seen on amazon with a backlash nut.