A 12" speaker will have a deeper bass, but I think the cabinet having a closed back is more important than the difference between a 10" and a 12" for getting a tight enough bass in your guitar tone for metal. Having said that I still wouldn't use anything smaller than a 10", not that I've tried it, but unless someone could really prove me wrong that any smaller of a speaker could get good bass at higher volumes I'm certainly not willing to try that one myself. So a 10" will be fine, I've never played my amp through a 12" speaker, but it sounds awesome for metal through the 10", maybe a 12 would be a bit better, but you're certainly not suffering with 10" speakers.