Ok, so after doing 2 gigs this month i'm no closer to solving my problem. My high gain block is a 5153 Blue and a 5150 Red, and if i stop playing in between chords or during a solo, hello low mid howling feedback sound! Those MV's are backed off between 4 and 5 as per your recommendations, and my Input gains back to around 5 or 6, which is ok for an AC/DC tone, but its just not enough vibe and gain for the Zakk Wylde / De Martini gain vibe i'm going for.
My real 5153 50 watt head gives me that vibe straight away at high volume with NO howling or feedback. My workaround for the Axe FX is surfing the my volume block with a mission pedal during the gig to stop the howling , but its plain annoying and distracts me from actually playing guitar. I have a gate block, but in order to get rid of the howl it also takes away all sustain, chokes the crap out of the sound.
I can also solve the problem by backing off the volume anywhere in the chain (Amp Input Gain, Amp MV, Amp block level, Matrix power amp level, its all the same), but then the high gain tones are not loud enough on stage. Turn up the matrix, howling returns!
I tried lowering my DC-30 level and levelling up the 5150's, made no diff, the high gain tones just wont behave at a decent gig volume. In the studio thru my Focal speakers, never any feedback. For cleans and Mid gain, i'm using the DC-30 amp model (Matchless), in the same gig preset, and it sounds amazing loud.
At this point i'm scratching my head thinking that the axe fx for live use on stage is no good for high gain at a decent gig volume in a rock band. Tonights settings were Axe Out 1 on 5, Matrix up around 75%, Axe VU meters always below zero, Guitar is a Jackson Adrian Smith USA model with a Suhr Aldridge pickup in the bridge. I'm happy to post up the preset if anyone wants it. It never feeds back at home in the studio, only at gigs with the matrix and the FR212.
I really want to get to the bottom of this and sort it out, or i might just have to ditch the axe and go back to the EVH stack, its been driving me mad for week. Surely i can get this rocking?