slayer do use marshall, king uses a jcm800 and hanneman uses a jcm900. and they sound ace. line 6 amps are nae that good. one option to get slayer tone is to go for a jcm800 and mod it to run at 50 watts so you can crank it, or get a powerbrake for it. powerballs are very very good and you can get a slayerish sound out of them.