I will check them out - never heard of them before.
I agree that metal music is almost never "rooted" in classical music - rather it is almost invariably rooted in metal.
To take Malmsteen as an example, while the licks and a few techinques (such as cycle of fifths) are taken from (mostly) Baroque music, the music is usually in a hugely simplified song format.
The concept of development is all but lost - and that was a key part of "Classical" music (to use the generic umbrella term!).
Avant-garde rarely truly exists - I'd like to hear some that isn't based on improvisations that use an "old" mode.
Covering Vivaldi isn't new - Dutch band Ekseption were doing that in the early 1970s.
But this is an interesting topic - I tend not to take Classically influenced metal too seriously, as it tends to be metal that sounds a bit Classical rather than truly Classical that sounds metal - but if there is any that really does root itself more in Classical than metal, I'd be interested to hear it.