The EHHA Rev A is, obviously, a revision of the original EHHA amplifier. The basic design of embedding a low voltage tube as the differential amplifier front end of a solid state amp remains the same, along with the symmetrical drive configuration. The only significant amplifer change has been to add emitter follower drivers before the output stage to get more bandwidth and to simplify the CCS on the input stage by using a CRD instead of a BJT.
The more important difference is on the board where each Rev A board includes both a complete rail and complete heater supply eliminating the need for other external supplies. Many thanks to the head-fi community for refining the board design and, particularly, the heater design to fit the back of the board.