I get the first half but not the second. You do realize that most racks use flexwatt to heat the plastic tubs? That's why the tstat is so important. Using a uth with a plastic tub is fine with a good tstat.
As for the heat lamps and stacked tubs. I have used a lamp to heat one tub but never a stack of tubs. I think your best bet would be as I said and heat the area the stack is in. Maybe a small closet with a small heater.