[35Z5]

Adding a resistor to lower B+ likely causes additional over modulation... Yet to see a instance that it did not... 

Modulation linearity with a pentagrid mixer/osc type will be poor at best... Not so bad for a kid yelling into a mike that his buddy next door is trying  to receive, usually unsatisfactory for music... Max usable modulation level for music probably 30% at best...  

Add a 50K-100k pot to input to set gain level...

For drift, those old wax osc coils are famous... Removing and baking at 170-180* for a few hours will help, but ultimately it needs to be in a stable temp environment when in operation...

I'd switch to transformer operation...  The LP-427 could be configured to provide B+ and supply for tube filaments... Use one primary as B+ supply and half of secondary to supply filament voltage... Would still require a resistor but can be far less value and wattage, probably become only slightly warm...

http://www.mpja.com/12V-1A-Center-Tapped.../27838+TR/