little transformer overload

i want to use 2xecc99 as power tubes in a little amp, but for the heaters they require 2x800 mA, so 1600 mA, and my 6.3V output is rated for 1500 only.
So i tried, the amp works fine but the tranny is warm, not hot but warm. I can touch it, it does not hurt at all.
But is it that bad to have 6.66 % overload on the heater winding?