So, just a little technical explanation if anyone is interested. In the Megatron design, the driver 12AX7 and EL34 outputs operate within a global feedback loop with the gain set by the ratio of R21 (33) to R11+R12 (23+24). The operating conditions for the 12AX7 are set by the constant current loads Q1 and Q2 and the grid-to-cathode voltage which is set by R11.
If you look at the tube characteristics, for a set grid-to-cathode voltage, as the plate current increases the plate voltage will also increase. However, because the plate current runs through R11, this makes the grid-to-cathode voltage more negative, which further increase the plate voltage. The result is that the plate voltage gets closer to B+, which can result in premature clipping of the driver stage. So in order to increase the plate current without increasing the plate voltage, we must decrease the grid-to-cathode voltage. This is done by decreasing R11. Note that this will also increase the overall gain and decrease the closed loop bandwidth, however, since R11 is only 1/3rd the value of R12, a 10% change in R11 will only result in a 2.5% change in gain and bandwidth. If we wish to maintain the overall gain and bandwidth, then we need to increase R12 so that the sum total of R11 and R12 are unchanged, .e.g if we decrease R11 to 2k, R12 should increase to ll.3k.