I know that you already know about the G12K-100 - go for it! They're really superb for mid-range grind and handle high power situations extremely well.
Pete mentioned the EVM12L - I have 2 of the black labels (300W power handling each) coupled with 2 G12K-100s in a 4 x 12, and I couldn't be happier with that cabinet. I think it's a great pairing that maintains a lovely clean sparkle but also facilitates really brutal grinding metal, and I play in drop A or, for one new track, drop E (E-E-A-D-F#-B). No problems at all in the low end!
If you can't/won't stretch to the EVM12L (black or not), I'd stick with G12Ks. Alternatively, I have 3 spare Peavey 5150 Sheffield 1200s (I put one in a Fender Princeton 112 60W solid state amp which is now absurdly loud), which are also remarkably good speakers for heavy music and also hold up the cleans surprisingly well. They have a lovely frequency response, and I'd put them pretty close to the G12Ks for quality of sound for metal.
Roo