the AD200 just died
The symptoms you describe suggest the heater circuit failed. I believe there is a connection that is prone to failing on the Mark III. I believe it's an easy fix. https://www.talkbass.com/threads/he...hum-but-no-power.1418800/page-2#post-23052637
I haven't had any problems with mine but I haven't gigged it. It's not dark sounding through either my GB 1288Ts or EV TL405s, so perhaps your perception is due to voicing of the OB810. Max out the master and mine sounds and responds very similar to a blackface Fender Showman (or Twin Reverb).
I have a Matamp GT120. I believe the vintage GT200 used pretty much the same preamp. IMHO, the AD200B is definitely more Fender than Matamp.