a used ENGL Fireball!
If you just want an awesome metal tone, with no constant fiddling with loads of channels and settings etc then i can't recommend it enough - i loved my 60w version. It did the melodic death thing very, very well, and was capable of very tight high gain (and ultra high gain!) sounds. It was also good with the gain rolled down for more easier going rock sounds.
Downsides? Shared EQ, 'ok' clean sound (not an issue for me!) and not that flexible outside the realm of metal. Oh, and at the time it was quite hard to find one, but that was when they were new out.
I sold mine to a guy who played in a technical death metal band, the other guitarist used a 5150 and they had a fantastic sound. Randomly, Michael Angelo Batio also used my Fireball setup for a local guitar showcase he did, and it sounded fantastic doing the whole shred thing aswell ;)
I don't gig so the Fireball was overkill in my house, so i eventually settled on an ENGL 'Modern Rock' E530 pre-amp which runs into my PC. This pre-amp also has killer high gain sounds and would be another option for you -- you could probably get a used E530, cab and something like a Peavey 50/50 power amp for around your budget and that would be a good rig for all things metal.
I have also owned a Screamer combo (sold it to someone on here in fact!) and it was more versatile than the Fireball but like you i couldn't get 'that sound' out of it.
As others have said I would also recommend cranking the Screamer through a cab - that way even if you don't like it you still have a cab which can be used for whatever you get next.

And finally...
I swore by my Weber Mass Attenuator - it let me crank the outputs on the amp up, but dial the volume down, so i had all the valves firing hot but without being deafened by the volume - it's how i played at home with the Fireball, and a Marshall JCM800 before that. I got the 50w version with the extra tone controls so i was able to shape the tone to minimise any colouring the Mass may add (which was barely noticable). I spent a couple of months re-searching attenuators and came up with the Mass fitting my needs the best, and it was switchable so much more flexible than the Hot Plate and some of the other ones out there.