Try pulling the 50L6 out. That will kill your filament current (breaks the string) and your B+ (no rectifier filament, no output). See what happens. If the resistor still smokes, then there is a short somewhere. Go over it with a clear mind and a bright light - look for dripped solder, bent tube socket pins touching, capacitor leads touching something, etc. It's easy to bend things just close enough to touch while you're working. We have all been there, and done that. Don't worry - you'll find it.
And the resistor didn't smoke with the 15W bulb because the 15W bulb has a much higher resistance than a 40W bulb. It couldn't get enough power to smoke.
Can you post a photo of the underside of the chassis?
-Ian