I've modded two Shivas (a head and a combo) lately, so I can compare Diezels to it: the Shiva is a pretty traditional amp (which is cool for some guys and not cool for others). The Shiva's clean is off a Fender circuitry and sounds like it (if modded a bit), esp. in the combo version with reverb. The second channel is off a 800s Marshall, and because its a Bogner, it is less bright, more dark and smoother than an 800s. The boost of it - well - is just a boost. Turns muddy too fast IMO.
A VH4, although coming from the same "age" of amp building, is much more modern sounding. The clean is more HiFi with a better mid control (it has a mid pot
), channel 2 is comparable to Shivas crunch, but with more oommmph, channel 3 is the Diezel trademark sound, completely unavailable in any other amp (and eats the Shiva for breakfast), while channel 4 goes into gain territories where the Shiva just dreams of, completely unreachable. Feature wise the Shiva is also traditionally built: VTLs, a very old school serial loop and two eqs for three sounds. While I like certqain aspects of the Shiva's traditionalism, the VH4 wins in all areas: much better loops. midi, 4 eqs for 4 channels, a much more modern sound.
Don't get me wrong: if you are a more bluesy player the Shiva might be your thing - but then I would compare to it to a Schmidt. And again: the Diezel "wins" - the Class A clean with tube driven reverb sounds even better (in a traditionalist way) than the Shiva and the crunch/über-crunch of Schmidt is much more versatile than the Shiva ever could be. the loop is switchable (and able to run series or parallel), the amp is much better controlable.
Just my opinion of course.