In the 300B rev 2 the changes are upgraded current sources and extra current sources in the driver stage.

The plate load CCS's were upgraded to Rev 4. This added noticeable depth and detail with the background getting farther back and blacker.

The additional CCS's are loading the MU output of the driver CCS's. This increases the current available to drive the 300B's.

In one of the upgrade rounds I used resistors to increase the current. This was a mixed bag. Dynamics got a little better but low level detail was worse. After some thinking it became apparent that the resistors had defeated my ideal of constant current operation. I was driving a healthy AC current into the power supply. Next round was the first use of the battery biased mosfet CCS. This was a true improvement. Dynamics improved and the high frequencies got a smoother sound with more finesse.