I have the the Shuttle 3.0/8 combo. I've not tried combining it with a 1-15 cab but am considering a 1-12. If the 15 can handle the full 300 watts at 4 ohms, I bet it would sound great.
The VT is definately cool, although I find the Shuttle has a pretty warm tone on its own, surprisingly so in fact. I mostly use passive Fender
Js with flatwound strings, lately playing alot of Hendrix, Stones, James Gang, various blues styles.