I like the Shiva. Even with 6L6, KT88, etc.... It's still thick and forgiving. Mids aren't as fat maybe as the XTC and the amp is not aggressive like a Marshall or metal amp.From my experience, my KSR Gemini is the overall best amp I've ever owned. My experience with Revv was not a positive one at all. I felt the amp was bland and lacked any sort of character. Unfortunately I have no input on the Shiva, but have owned a couple Uber's over the years. Based on clips though, I feel the Shiva is what your describing.
My Archon is way more aggressive.
I only wish the Shiva had a footswitch to control the effects loop. That's important to me.
I can manage but it's a pain.
I contacted KSR and they recommended the Juno as the most forgiving and fun to play. Gemini and other models as sounding slightly bigger but more stiff.
Colossus seems awesome with all the channels and options.