I dunno. I am kinda familiar with this and noting clicks in my brain here.
I can tell you this, the 20mV or whatever highest sensitivity range is THE reference. Get that flat first. After that then deal with the attenuator(s).
It uses those little modules, they probably plug in. I got a couple around here I suspect, I am waiting for an intermittent to start popping up again and once it does I am going to switch them channel to channel.
These 7000 series plugin jobs do make for a little bit different experience. First of all they don't give much on what exactly is supposed to be coming our of the plugin to the mainframe. Like amplitude. However it stands to reason whatever the level the frequency response should be flat. you should be able to see it at the plugin toward the back. If that is flat it is the mainframe. if not, make it flat.
From there, all the ranges. there are screw adjustments on those little modules, it sounds to me like there was something wrong and someone tried to adjust it out and frigged it all up. It can be corrected but if that is the case I would like to break the guy's fingers off. There should never be a reason to adjust those little modules unless replaced.
Anyway, that's my thoughts on it. Get that most sensitive range to produce flat response. All those modules are our of the circuit. Don't do anything else until you get that response flat. THEN screw with everything else.
The signal you need should be at A11 and B11 in the back, differential.
I can see the picture but don't have a unit in front of me, though I just worked on one last week, I can't say for sure but I think those little modules plug in. That means you can switch them channel to channel. Once you are flat at the highest sensitivity you can tell which modules are the problem. To make sure switch them channel to channel, they should act the same. If not, you KNOW someone was in there diddling without knowing what they're doing.
i have dealt with that thousands of times on various types of equipment over the years. If that is the case it is a bit harder but it is not the end of the world.
Poke around, especially at that A11 and B11 and tell us what you see.