Nice I will take two, just dont expect the 300B to lasts long.
Steered Current Source Technology (SCS)
The 91E uses Western Electric’s patented steered current source technology. The unique parallel feed topology contributes half of AC current to the plate (anode) of the 300B by modulating quiescent current. As a result, half of the power dissipation occurs in the tube, allowing for a greater output power of 20 watts per channel.