Frontload for me. And congrats on the Herbie very versitile amp. Everybody always thinks metal when they hear Herbie but it does way more than that and it does it Well.
I recommend Diezel FL412 with V30's all the way I love it! Anyway I have no experince about G12K100 speakers but I'm pretty sure that they are little too sterile for my taste...
Go with a front loaded cab, why you ask? Because they project better and produce a tighter sound where a rear load is a bit darker with more sag in the sound.
As far as speakers, I'd ask if you could order it with an X of V30's and K100's. I have all 100's in mine and am wanting to do the X pattern myself. I wouldn't go all V30's though, K100's are very tight and clear.
The only thing sounding better than the FL K100 with the Herbert, was, when we had a FL K100 and a FL V30 together on the map.
Therefor, my choice would be a K100/V30 mix, but i wouldnt go strictly X-pattern - consider mixing them left/right (f.e. left: V30, right K100), so you could also use one speaker-type exclusive (in the Studio for example).