FAST AND DEEP: I run these BOTH OFF.
For me...fast ON clips the good signal short...and I don't like it.
It just simply "messes" with the sweet high conductive sounds warble... and when you are hunting with some iron mask it cuts that sweet sound short which is very detrimental when listening for that signal breaking through the silence of a junk or iron null.
Even in areas of dense trash I still leave it off and will sometimes STAND COMPLETELY STILL...and not even step forward...and not even swing the coil.
I absolutely "creep" the coil and don't even consider it swinging...just standing still in one spot while slowly "scanning" the ground with the coil.
Only after I feel I have given it a thorough and complete job of "trash sniffin'"...will I take a step forward.
For me deep ON distorts the signal too much...and I don't like it.
Deep ON simply affects weak target signals and "enhances" them while higher gain settings actually amplifiy the target audio response...so with a setting of 10 I am hearing said "weak" signals as loudly as possible with the SE.
I already have my gain jacked to 10...so there is no need for me to have deep ON anyway.
Most of my sites are devoid of any good signales at depths of less than 10" anyway...so I have absolutely no use for a modulated audio supplied by running lower gain. Again...I want to hear those deep weak signals as loud as I can.