Internal Dough Temperature in Sourdough

My internal dough temperature for sourdough is generally in the range of 68-73 just after kneading. I realise that many a dough have a desired temperature closer to 80, most commonly I've seen 78. My kitchen is generally about 60 degrees though. I've used water up to about 80 degrees, but still haven't broken the mid-70's in my dough. How high can I go with my water temperature when mixing it with a levain? Would temperatures up to 90-100 be detrimental to the beasts in the levain? I generally will mix the water with the levain first off, then add flour. Should I just forget about the whole thing and keep up my extraordinarily long fermentation times?