Yeah I noticed the layout was definitely different. I had the gain almost full out and the volume at pretty much 3 quarters. I had the EQ all around noon though as I didn't know about the tone stack. Just so use to my Ampegs where I can just set everything at noon or around there and it just sounds good and loud lol.
I'm really surprised to hear that your volume levels didn't seem adequate with those type of settings. I hate the claims of "something was wrong with the amp" just because someone else's experience is different from my own, but in this instance it really doesn't sound right. Yes, your Ampeg does have more volume on tap and more mid control capability, but that amp should have been a snarly, sick mess at those levels. The only time I've experienced severe volume output dip in the AD200 was when I had power tube failure.
I've got a 4ohm, 2x15 Sunn cabinet (150W) that I've played my AD200 through before, and I'm able to get massive volume out of it. I don't crank the gain because power amp drive sounds better to my ears that preamp tube distortion (YMMV), but with gain at around 11-12 o'clock I'm only able to get volume at about 9 o'clock before I'm shaking things on walls. At your settings, I would have fried speakers.
Do you normally play super distorted tones? Because that's really what you should have been getting at those levels.