hahaha rectifiers were designed to play grunge just as grunge was dying out. Mesa got lucky when some heavier players discovered they could be used for metal. And yes, they are great for metal. Look at bands like Skinless, cannibal corpse, satyricon... they all use rectifiers, and are definately NOT rock. Not all metal is tight technical stuff.
5150 was designed for van halen, but pulls off AMAZING metal tones. It is a metal workhorse.
bands like Between the Buried and Me, Splatterhouse, Dark Tranquillity, In Flames, Dismember, and Necrophagist all use the 5150 pretty consistently.
Now for ams that have been designed for metal: Engl Powerball, Engl Invader, Engl SE, Engl Savage.... hell even the Engl Blackmore can do metal, and they all sound awesome doing it.
Vader uses powerballs, and SE's, dimmu borgir uses savages and SE's, Zyklon uses powerballs and Savages... cannibal corpse even recently added a powerball to their mix.
Also, the framus cobra is CLEARLY a metal amp, and definately pulls off the oldschool brutal, as well as any new tight and technical stuff.