Harvdog...in reading posts and articles by Tom Dankowski, who has worked on CZ's and helped design the CZ3D, he states that the 'nickel window' range of a CZ is the most accurate of all detectors. What's good ( and sometimes bad depending how one looks at it) is that the nickel gives a high tone. You have to read the meter to determine where the needle is on a high tone hit, but when the meter reads nickle, it almost always is.
I think a lot of tone ID detectors have a nickle hitting in pitch somewhere close to pull tabs/beaver tails.
So you can dig a lot of nickels without digging a lot of junk with a CZ just digging high tones. The problem with the older CZ's like the 6a is that some older nickles and Indian head pennies ID as mid tone. That is where the CZ3D comes in because it brings these coins, along with some gold, into the high tone hits when in enhanced mode. Contrastly, it also brings some trash into that high tone too, so that is why the caveat with the 3D is to use it in enhanced mode only in older, less trashy sites.
The 6a is my favorite detector of all time. Once you learn it, you will love it. Just make sure if you hunt in discriminate mode, to always use level 0 so you can hear ALL targets, including iron. Once you get used to that, things will begin to click and you will realize how to tell false hits of iron from the real good targets. If you run discriminate higher than 0, you will not hear the low iron tones and if some of the iron bleeds into the high tone, that is all you will hear and think it's a good target.
I never use my 6a in discriminate mode other than setting 0. Even if the site is littered with iron. I have pulled a lot of silver coins right next to iron with the small coil ( and stock coil) since I can listed to the iron and when I get a repeatable, mellow high tone hit, I know it's a coin.
Have fun with it...you will find out it's one of the best detectors ever !
JC