I would have thought that 66º was good for your plants,
but I could be wrong.
I was just reading arcinNsparkins "Lemonstarhempwarp" thread,
and he is running his bubble buckets at 50º.
I think you might shock your roots,
if you changed the temperature very fast.
I guess the real question is,
what is the temperature for the most DO?
I imagine that altitude plays a big factor in that equation.
The higher you are above sea level,
the colder you need to maintain,
to have enough DO.
Most of us can get by with 66-68º
because most of us
are not over a 1000 feet
above sea level.
.