I've been tweaking the bloom workflow ( luminance calculation, outputting threshold values to texture fraction size of main target, then recombining the original target with gaussian-blurred luminance target, tone-mapping, contrast reduction, gamma correction to final main device) for awhile now.  I've finally got it right.  Tonemapping is not an exact workflow.  There are many different variations of tonemapping operators to choose from.  I've been experimenting with several different tonemapping workflows.  Some are very complex, explicitly using White and Exposure variables, others are much simpler and get a pleasing result.

The tonemapping operator I wanted to use works great--the explosions are HDR and very bright, with lots of definition--but the background (everything that's not an explosion) looks comparatively very dark.  Varying contrast reduction and gamma correction doesn't quite fix it.  So this video output represents a visually pleasing tonemapping fallback operator--that is also a breakthrough result.  Bright explosions, and easy to see the background.