It's not because it can Profile an amp, or tone match tones, etc, etc...it's the best because it's sonically beautiful.
It's all very well Profiling an amp or tone matching, but if the KPA brain/algorithm didn't have the ability to turn all the Profiled information & audio frequencies it receives into 'cut glass' audio then it would all be pointless.
I keep reading about guys saying they landed on a Profile/Rig/Patch and ended up playing for hours, it was addictive.
Thing is, it's not a particular Profile they're talking about, or particular studio monitors/guitars/pickups, we're all having the same experience using different Profiles & gear, so it must be the Kempers unique interpretation of the info it receives.
I asked a top studio engineer why his Access Virus Synth is so special and unique, he said he doesn't know, he just knows after going through dozens of different synths over the years he kept the Virus because it sounds awesome.
Of course the Line 6 and other engineers know what they're doing, they're geniuses...but they can't get their programming to interpret into the audio that the KPA produces.
Play the KPA for two weeks and then go back to your HD500, you'll hear what I mean...and it's not because the KPA has more expensive 'parts', it's because it's programmed differently.
Sure, these other engineers model every aspect of the amp down to the transformer and thickness of the speaker wires, every control apparently works exactly like the real amp controls, well, what's the point if the end result doesn't sound as good as the KPA end result?
And, by continuing to add more tweakable in depth features/parameters they think that the audio improves, it doesn't.
The "feel & dynamics" might improve, but the audio remains the same...if an audio frequency sounds like a cocked wah you can add all the feel & dynamics features you like, it doesn't get rid of the cocked wah effect.
Morning cup of coffee thoughts. 
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