Well, I guess I will share my experience digging deep coins for you. I have dug many coins now in the 9 inch range, and a few that approach 10 inches. That being said, I should also mention that I have dug thousands in the 6 to 8 inch range. I have used the MXT,DFX, and V3 and gotten what I consider to be deep finds. Midas has given you the simplest answer there is. That answer is experience teaches best. We can accelerate the learning curve by taking advantage of the forums and books and hunting with a buddy who has experience. The later being one of the most advantageous in my opinion.
As mentioned previously the pin point volume will decrease faster than the discrimination mode audio responses. The coins that I have found in the 8 to 9 inch range have in many cases presented them self as only a light variation in tone in the pinpoint mode. At this point I would like to recommend that you get a good set of headphones. They do help significantly, and help you focus on the task at hand. On more than one occasion I have had to start the pin point procedure over several times to get a good response from a deep target. It is by no means always obvious. Pinpoint does get difficult for me after 7 to 8 inches sometimes, especially if I am dealing with EMI of any kind. It gets real light and wispy sounding. I have my volume set fairly high, but I am a little hard of hearing. The higher volume made a difference for me. Experience is going to be your best friend. Get out often in locations that give you a chance to find those old coins. If your getting used to that penny you buried at 7 inches, bury another one at 8 inches and experiment. Trust me on what I am about to say...

If you can get a good hit on a coin at 7 inches in motion mode and pin point it, you have the ability to find a hell of a lot of old coins that are still out there just waiting for you. Take what you have learned and find a few coins at depth. I had a huge break through myself this last spring. I hunted right next to a guy who pulled coin after coin. I watched him closely. He moved slow and deliberate, and one more thing, he also happened to be hunting where the coins were. Make sure you hunt where the coins are. I went back the next week and followed his path through the park. It was one of the best Silver days I ever had. Martin, with your steadfast determination and persistence I know you will be digging deep coins in no time. I will do my best to answer questions for you, but get out there and hunt with the new found knowledge you have acquired. Then make sure and share the results. Little steps my friend.

Jack