There isn't much of anything I can contribute concerning the FX reference accuracy or stability. It's an order of magnitude better than anything I own. Something that does interest me is noise. I know that that's been investigated too, and it's darn good. Still, it might be interesting to do a visual comparison with some other references. For my audio work I have a Tek 1A7A plug-in that goes down to 10 uV/div. Also have a similar one for my 7000 mainframe, but the 1A7A is my "daily driver." Warning- all my photos are hand-held and you'll see some intensity changes from the trace speed and shutter speed.
First, the 1A7A with a shorted input:
Next, the FX reference:
Next, my Fluke 731B reference:
The 1A7A is a 1 MHz plug-in and the 731B has some HF noise. Here it is with a 10 kHz lo-pass filter:
The real surprise is how bad my 731A is. That version had no voltage regulator and no internal floating guard:
Finally, the Mini-Metrology LT1027 based reference:
Pay attention to the vertical sensitivity labels, as I've used different scales as needed.
I've just completed a voltage regulator for the 731A. That helps a lot, but it still suffers from no internal guard. More on that later. I'll post some voltage comparisons shortly.