#1
i have a seriously terrible problem with my marshall AVT150. when im playing with my band and i turn the volume up, it starts to produce these eardrum-bursting squealing noises that seem to respond to the way my pickups face the amp. my seetings are, gain at 3 o clock, bass at 2 oclock, mid at 11 o clock, treble at 12 oclock, and the mid scoop switch on and presence at 3 o clock. i have tried everything, from standing as far away from the amp as possible, to changing the settings so it doesnt feedback so bad ( and when its not squealing, the tone is terrible). Can anyone help me? my rig is : Ibanez RG2550E (DiMarzio pickups)
Marshall AVT 150

that is all. Can anyone help me?
#2
If the tone is terrible as well then it might be the preamp tube needs replacing. Either that or a grounding issue, but since you mentioned you're getting worse tone out of it, that's a common sign a tube is failing.