Maybe Bob always disconnected battery so he did not notice this?

Also, there was a change in DSP amp in -98, after that it became programmable and actually uses DSP for frequency filtering. Earlier DSPs used DSP chips just for EQ and echo and filtering was done with regular op-amps before power amps. Maybe this programmability also changed how the car handles the DSP i.e. does it disappear immediately or only after the battery trick.

When I upgraded to DSP I made an adapter lead so that I can quickly go back to the regular BMW hifi amp setup. And during the test time I noticed that I had to disconnect battery after trying the DSP if I wanted to use regular hifi amp, otherwise HU would output fixed level. But when going from hifi to DSP I just needed to plugin the DSP amp, switch ignition to 1 and HU would output fixed level. I also tried to unplug just radio and navi fuses, but that was not enough. Looks like DSP info that the HU uses for output selection is stored to the car main module and that reboots only when you disconnect battery and wait few (5) minutes.

I also noticed that if you unplug the DSP and switch ignition to 1, the DSP menu selection goes to grey and after turning ignition off and again to 1
DSP has disappeared but HU still outputs fixed level. Only disconnecting battery made HU output again variable. This same thing happened with dash mounted Professional CD and also with BM54 navi radio.