If you want more gain, there is a 270k/330k voltage divider before V1b. You can clip in resistors parallel to the 270k to test lower resistances (higher gain) in the preamp. (A 1M across the 270k will be equal to about 212k. A 470k across the 270k will be equal to about 170k.)This micro bassman is amazing i used a p10r from jensen and it sounds great. It just lacks some gain despite having a 12ax7 (i use eh 7025) perhaps should i go for jj tubes ?
More gain could be had by reducing the 33k grid stopper at the input as well. You can go as low as 15k and still maintain good RFI filtering. I would probably just adjust the voltage divider rather than change this though.
From my experience, the JJ 12AX7 has a grittier quality than the EH. It may be preferred for high gain type sounds but not always. It shouldn't make much difference in gain.