Just my .02, but if you want crushing distortion like that, lose the EL34's and put some 6L6's in there. Since the amp is used, if it were me I would retube the whole thing just to be safe...if you just bought a used car you would change the oil and spark plugs, right?
As for metal tone try this...Channel3, modern, bold, diodes, bass 12:30, mids 11:00, treb 12:00, pres 10:00, gain 2:30, master to balance w/ other channels, output at whatever your neighbors will tolerate. Like most tube amps, the more you crank it up, the better it will sound.