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?
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.