Well, Bob...
Most of the time, I have kept deep on and fast off. However, and this might be my imagination, I've noticed that the recovery time to threshold after a target is detected, seems to be slightly faster when I set both fast and deep off. From the description in the manual, it seems to me that deep on is very similar to the gain control...working on enhancing deep targets to make them sound louder. So, I feel if I run the gain at 8, I shouldn't be missing any really deep targets. I noticed that switching back and forth from deep on to off after identifying a target doesn't seem to lose any depth. The deep targets still come through about the same.
So, from now on, its both deep and fast off, unless I'm in a relatively clean area that I think still hold some very deep ones.
Sensitivity: Although the auto setting makes for some really stable detecting, I try to use the highest MANUAL setting I can without causing a lot of falsing. I've switched back and forth (again, after finding a deep target) and many times, the signal is much fainter when auto sens is selected. So, manual sens for me, at the highest setting I can, without causing total instability.
Knipper