Yeah, the SL2 is definitely MORE "XTC-like" than "Shiva-like". But it can get close to the medium-gain levels of the Shiva too.
I really think the secret is those Chinese tubes. I remember replacing the stock chinese tubes in my Shiva with JJs, thinking that I "upgraded", and didn't like it at all. It didn't sound like a Bogner anymore. Then I tried EH tubes. Same thing. Threw the chinese tubes back in: bang, the Shiva was back. It was very strange.
Bogner isn't lying when they say that they designed the preamps around THOSE tubes. I think they realized that while they're not as rich sounding as some vintage preamp tubes, the Chinese tubes have a chewyness and a glassy spank that other tubes don't have. I think Bogner worked around these qualities and made amps that compliment those tubes.
Shiva vs. Mod50. Me and Matt did some extensive AB'ing of the two. And one of the things that finally made me sell my Shiva was the fact that you can get great useable sounds on all ends of the volume range. Not just cranked. The Mod50 sounds great even at lower master volume levels. In my opinion, for clubs that want to keep the stage volume way down, this is a must.
Don't get me wrong. The Shiva was pure magic when it was cranked, but I didnt' find THAT many real life situations when I could really crank it. And that was a big problem. The more and more I played that amp in real life scenarios, and had to mess with the wacky FX loop cables, etc, I realized how limited I was using that amp.