It's not good to run tube heaters that high, you can go lower than recommended 6.3V per heater (like this one does, with 4.5V per heater) but anything higher than 7V will reduce tube life noticeably. What you can do is put a resistor in series with heaters and run it of 18V, you'll get higher plate voltage which is good and you'll be able to tune heater voltage to 12.6V.
Let's see, if you start with 18V, minus 12.6V = 5.4V. From ohm's law you'll need resistance of approx. 5.6V/0.15mA = 37ohm. Approx power dissipation is 0.8W. I'd take 2W resistor of closest value to 37ohm. Any clearer?