I guess I just don't understand this obsession with extreme depth. Who wants to dig that deep on a regular basis anyway - lol.
But I agree with you the delta in depth performance should not be that great between the 9 and 11" Deus coils.
However, it is hard to do apples to apples comparisons at extreme depths (> than 7 or 8 inches) because there are so many variables that can affect the results that I think it gets pretty unscientific and subjective pretty quickly. At least in this case, you were using a consistent target benchmark (a clad (?) quarter).
Your post is a little hard to follow. Did you recover/detect a 12 inch quarter in the field with your previously owned 11" coil or were you using a test garden? Did you exactly replicate the ground conditions you had from the 11" coil (ground conductivity and mineralization, moisture content, coin orientation, coil height above the ground etc.) when you attempted to capture the 12 inch quarter with your new 9" coil? Was it freshly buried? From the detector end you need to be careful about hopping around the built-in programs because regardless of what they are named. For example, some of the preset programs incorporate a high silencer setting (i.e., higher than 0, preference would be -1) which can significantly affect depth.
These are the settings I would use to maximize depth, including discussions of how the settings affect depth performance (I typically start with Deus Fast as my base program and make my adjustments from there):
Discrimination - anything less than 10. Despite folklore, I have seen actual test garden demonstrations that show that discrimination does not appreciably affect depth until you push it well above 15 or 20. I personally like to run it full negative with version 4 (i.e., -6.4).
Tones - does not affect depth but I am partial to full tones because you can hear those high tone squeakers. If you have everything above Target ID 80 the same high tone (say in a 4 or 5 tone setup) than you can't differentiate those high conductivity targets. You want a dime to sound different than a quarter, if possible. Full tones gives you that.
Sensitivity - as high as possible without falsing. If you can get it to at least 93, then you have pretty much maxed out depth because depth pretty much plateaus out above sens 93-95.
Transmit Power - Set it to 3 if the soil is not mild to highly mineralized, otherwise 2.
Frequency - If you want to maximize depth go to 4 khz though that locks in Tx Power 3 which could be counterproductive in highly mineralized dirt. Let's face it, regardless how you set up the Deus, depth is going to significantly suffer when the soil is highly mineralized. Just an unfortunate fact of life.
Reactivity - Recommend setting it at 1 for max depth, if trashy bump up to 2 or 2.5 for separation. Separation/Recovery vs. Depth, no real free lunch there. If you want to max depth, set reactivity lower.
Silencer - Set to -1 (off) for max depth, some say you can get away with 0 with little-to-no depth loss.
Ground Balance - within 3 points of actual conductivity reading. If you want to maximize depth, push the GB reading such that it is 3 points less than actual GB reading. This will give you some extra depth at the expense of additional noise.
Also, no notch.
In terms of ranking them from greatest to least impact on depth (assuming a proper ground balance) I would say: #1 - Sensitivity (as high as possible) , #2 - Frequency (as low as possible), #3 -Silencer (-1/off), #4 - Reactivity (1 or lower), #5 - Tx Power (as high as the soil mineralization conditions permit), #6 - Disc (keep it anywhere below 10 and you should not impact depth).
Lay these "max depth" settings in, bury your quarter and then let's talk. BTW - the dime is usually used as the target to benchmark detector depth performance. Obviously, a quarter should be detected deeper than a dime. The point is that consistency matters when comparing coil vs. coil and detector vs. detector.
IMO the variables beyond the detectorist's control make this whole depth thing subjective.