The 20th Shiva is a great amp, especially with the KT88s. I used to think it was a JCM800; while it has some of that DNA, it is its own beast. Modern back end with a very full-spectrum EQ. Through greenbacks, it can do the vintage thing and through G12-65s can do a convincing Dumble.
That said, I really love '68 Marshalls and their derivatives. And that (along with a Fender Deluxe clean channel preamp) is what the XTC is. It has that in-between era give and take to the power section that I simply love, especially with good glass. Three channels with boosts and a switchable loop and standby all at your feet - it really is the best of all amps. I always want to find something better (I am LOVING the Dirty Shirley right now - which has a lot in common with the BLUE channel) but it always comes back to the XTC.