The noise gate on my YJM is now acting like another volume control. When I activate the boost, it lowers the volume as I turn up the noise gate. Using the noise gate alone does the same for any overdrive pedals I'm using out front. There's a guy who posted this exact same problem on youtube. His demonstration showed the exact problem I'm experiencing.
It's not a tube, having tried a variety of preamp tubes in all locations as someone suggested. It's in the shop now, but my tech isn't sure how Marshall will respond to this. New noise gate processor chip? New pcb board? Everything is really hissy now without the gate. Even with EPA turned up all the way, I can't turn up the front volumes without an incredible amount of hiss.
My tech also wonders what kind (if any) support Marshall will provide in the future. After these parts become obsolete, what kind of amp will be be stuck with? I'm re-thinking my purchase. Everything was okay a few weeks ago.