2A3 is properly too low for JM Lab 907Be, I am using a pair of 300B monobloack (8W) driving a pair of JM Lab 916 with great success. I think you got to have at least 8-10W to drive the speaker properly otherwise it will not enough power. 2A3 is only good for those speaker with 95- 100db sensitivity like Horn.
Do not really look at the sensitivity at all as a main consideration. Sometimes, I found it hard to drive some speaker eventhough it is 94db. Traditional wisdom saying that Horn and Old Full Range speaker like WE 555 or Altec 604 best suit 2A3. I had a friend who drive a pair of Altec 604 with a DIY 45 tube ampifiier, the sound is really old but of course it will lack high. DIY is really interesting, once you go into it, you will never look back at those famous brand who charge you an arm and an leg.
