I'm wondering the same thing...I've never tried a Shiva, I would imagine the 20th with KT88s would work well for Metal but I can't say. I've played the 101B several times and the red channel does a really good thick somewhat dark high gain. Rhythm and lead. That channel sounds a lot like the red channel on a EVH 5153 to me but the XTC can be thicker , have more low end in comparison. The voicing is very similar though.
I wonder if the KT88 Shiva does a similar type of high gain rhythm / lead but a bit more present and in your face. That could be cool.