The way I understand it is that deep on "enhances" ONLY weak target signals...and strong signals remain unchanged.
It is a fact that running both deep and fast off gives the best averaged results for depth...audio response...and target ID.
How we all interperet all of this is a matter of personal preference to a point...but the one fact is...is that running both fast and deep off is supposed to give the bast averaged, overall results to the 3 things I mentioned.
When I turned deep off... my deeper silver and injun' finds increased in a dramatic fashion.
I was finding a lot of coins in the 8" to 9" range...but could count the number of 10" deep coins found on about 3 fingers
Once I began experimenting a couple of years ago with different settings..and I'm talking 100's of hours of actual in field hunting....running fast and deep both off produced over 50 coins that year alone.... that came from 10" deep.
I would also add that all of these coins came from sites I had pounded for many previous years. I noticed it was just a nice overall combination of exactly what the manual said.
All this being said...I also run my gain jacked at 10 because I want to hear those deep faint signals as loud as I can. I don't want them "enhanced" by running deep ON...I just want to hear the original signal...whether it be a beep, chirp,,,etc...but I want to hear it as LOUD as I can...not "enhanced" as you get with deep on.
Again...this all comes back to whatever we train our ears to listen for is the combination of settings that work for us as individuals.
For me though...it makes no sense to have deep ON...then run high gain...especially when I've found that the signal quality sounds much better to me with deep off.
I used to run gain at 10...and also run both fast and deep on...and found tons of stuff as I mentioned earlier.
I felt like I took a giant leap forward in overall deep finds though...when I turned both off....and the amount of iron targets I dug also decreased when I turned both off.
It seemed to me that running deep on "enhanced" those deeper iron targets that fooled me into giving them a "shot"...then cussing as I plopped a rusty piece of crap out of the hole
A lot of people like to run lower gain so they can tell the difference between deeper and shallower targets. I like it high though...heck at my sites there's nothing left at depths of 8" or less anyway

...so the last thing I need is to differentiate between deep and shallow targets. I want to hear those deep chirps as best as I can...and not "enhanced" by running deep on. This way I feel my ears are hearing the "original" signal as loud as I can...and I can count on my experience to make the decision on whether or not to dig.
It's all a matter of personal preference...and we can all learn from what someone else is doing...then decide whether or not we want to incorporate it into our own settings
