On the 800 you can adjust the threshold level and you can also adjust the threshold pitch.
You can raise the threshold from silent at 0 to higher areas where you can hear it and also adjust the pitch to a customized frequency for each hunter...if they want to.
There are a few things you can do with a thresh tone set where you can hear it like reject targets and The 800 will blank out if you roll over them.
Kinda like notifying you about targets without actually sounding off on them if you hit one.
The pitch adjustments are nice if an owner has hearing issues or just like listening to thresh tones different than factory settings.
As useful as these can be to some owners to others they are totally ignored.
The 600 has the threshold too but it is set at 0, (silent), and you can't adjust it nor can you adjust the pitch.
It is all preset.
At least I think that is how all this stuff works.
If you are looking for a silent thresh using either at 0 a guy like you is good to go.