Hey guys, I'm sorry for yet another 'what amp' thread, but I really need some help here. I don't know much about amplifiers apart from the difference between solid-state and tube amps, and I'm looking to get a tube amp sometime soon.

I currently have a Randall RX20R which I use for practice only, and I want to get a new amp that will suit my needs. I'm looking to play metal mostly, and not much of anything else, and I have about 650 USD to bust. I would go out and try all the amps i could, but the range in the place i live in is pretty much limited to practice amps only and the really expensive ones, so i'm going to have to order online. Could you guys recommend me some stuff?

I'm using an Epiphone Explorer with an Ibanez Smash Box distortion pedal, and the sort of tone i'm hoping to get is post-black album Metallica. I know this is a really nooby request, but if someone could also educate me a little on tube amps and amplifier heads and stuff I'd really appreciate it. Cheers!