The AT Pro's 9x11 coil will fit it. Depth is really not much more, maybe an extra inch or so. I have the AT Gold's 5x8 coil on my Pro and I love it. It makes you slow down a little and allows you to get into tighter spaces. I still dig bullets at 7" - 8".
The optional AT Pro concentric coils will fit but realize what you gain in depth you end up loosing in the end with coverage. Remember, a concentric coil detection field is like a "V" in that at it's deepest, you only have a spot about the size of quarter that will produce a signal. Your sweep pattern will need to overlap significantly to improve your chances of finding those really deep signals. With the DD coils you have essentially a blade detection field from near the front of the coil to the back. You may loose an inch or so in depth with the DD coil, but the chance of actually hitting a good target should be much better.
