I know we're talking about the AM503B in this thread, but since I have it set up I thought I would share what the drift looks like on the older AM503. Same test scenario, and using the same A6302 probe.
The AM503 is an all analog implementation, with front panel buttons and knobs for manual degaussing, balance, and DC level. The DC level requires constant attention, even in short time spans, which is why the AM503B is such a pleasure to use with its AUTOBALANCE button.
The amount of drift over a 30 minute span is much improved in the AM503B by around a factor of 3 or 4 in these tests, as is the output stability for a minute or less.