I've been depth testing the 9" coil against the 13" for the D2 today.
The 9" is a sweet coil. I was in the farmers field with a couple of dimes. One was buried 8-1/2" and the other at 10-1/2".
Both were compacted in lifts. The soil was just a bit moist and medium mineralization. Very little EMI that I heard.
The 9" coil hit and ID'd properly (91) the 8-1/2" dime but won't get the 10-1/2". The 13" coil hits the 10-1/2" dime with the proper tone but the numbers were jumpy. It hits the 8-1/2" dime and ID's correctly, loud and clear.
I'm so glad that the bigger coil is a tad deeper, Whew! . . . I wouldn't have been a happy boy if it wasn't. That bugger wasn't cheap.
Swung the bigger coil after that testing, in an area that I hunted many a times, same field. I was digging deep poultry bands . . . thought I cleaned them out with my other detectors. They read in the low 60's but so do 1/2 dimes and Trimes . . .gotta dig em. PITA!