I got fed up with loitering on the Line 6 forum waiting for news about an update so that I could make the HD500 sound better, because it just sounded bad to my ears. I gave up and bought a Kemper.
If there's something missing the tones of the Kemper, buy something else. Whether that's a spring reverb pedal or a different amp, do it. There's no point in sitting here waiting for Kemper to come up with a solution to your problem.
POD HD 500 has two spring reverbs simulations, they're pretty good too . You can have a pod for $250 on ebay. Worth every penny if you ask me. THe drives are honestly better than the drives in the KPA, and you can use it as a controller to controller the KEMPER while using spring reverb too.