I've been inside the Kemper Stage and you can easily fit a 50w poweramp in it, but...
How about Kemper bring out a little external poweramp box similar to the Hotone Loudster?
It would be extremely easy to manufacture considering Kemper have a good supplier of their current Poweramps, it would take care of all unpowered Head and Rack and now Stage users.
Imagine something like this, with or without a volume knob and possibly in different power output options.