Create an env_tonemap_controller in Hammer, and set its name to something like "tonemap". Then compile the map with HDR. Once you have loaded your map in-game, open up the developer console and type in "ent_fire tonemap <input> <value>". For <input>, use SetAutoExposureMax, SetAutoExposureMin, SetBloomScale and SetToneMapRate, and for <value>, use whatever number you want to.
Experiment with these settings using different values until you get the result you want. There's a lot of trial and error here, and you'll most likely not get a good looking result from the start.
Next, go back to Hammer and add a logic_auto entity that fires the same inputs to the tonemap controller you did earlier with the values you found gave the best result.
This should help you improve your HDR lighting.