There are a few specs that are mutually exclusive, and you're basically ruling everything out.
- Doesn't have to be a very clean 6.3v, as long as it stays within 5% tolerance
How accurate must the rail be over load? Is that still 5%? This is why voltage regulators were invented; they provide a constant output voltage given a sweep of voltages and load currents.
A diode, resistor or any passive dropper "solution" have rather horrific line/load regulation specs, that will likely surpass 5%. A diode can drop anywhere from 0.3V up to 1.0V depending on the type, the load current, etc. These rules of thumb can work, but you have to remember these will only work for 1 load current and also for the particular diode you have chosen. Same really for resistance; 20 ohms may work at 12V, but maybe not at 9V.
Is there a large difference in load current between a cold and a warm tube?
But then you also require:
- Cant be a switching solution (so Buck converters are not an option)
- Can't evacuate a lot of heat (small closed enclosure)
Any linear regulator/solution will dissipate (excess) voltage x current as a heat power output. 2W is plenty of heat, especially with no heatsink on a TO220 package. A switching regulator is a better solution, but I understand for logistics reason you can't use it.
Any "dropper" circuit using earlier said components will still dissipate a similar amount of power, however it may be spread across multiple components. This can be an advantage and disadvantage; i.e. more components to cool (if each of them are running excessively hot) or multiple components to spread the heat.
My advice: get a switching regulator OR a linear one with proper thermals (heatsink/ventilation). If that will not last, you could bodge something together but only expect it to work under 1 set of conditions.