Do you have the powerstage pedal or the bigger one? The pedal looks small enough that I would just save the $400 if you have that one. That said, being a new Kemper powerhead owner, I'm amazed at the compact size of it. The poweramp sounds great to me so far. If you really want to find out, you're going to have to get the powerhead and compare for yourself.
Having only heard the Kemper amp (and plenty of other power amps in different situations - studio monitors, etc.), I would argue that the sound differences are probably subtle enough that it's not a big deal. Some of it will be preference and use case. Audiophiles go back and forth on what sounds better, this amp or that amp, this DAC or that DAC.... once you're spending a decent amount of money the differences get smaller and smaller and it's all preference and what fits your needs for your set up.