20) I've found loads of modern coins, what do I need to do to find older coinage like silver or wheat cents?
This is a tough question, and is not so much directly an issue of the MXT, it affects all detectors. Metal detectors have been around and in use for a lot of years and just about all the well known, older sites have been hit, most have been worked pretty hard. I once found a mercury dime one inch deep in a well known local park, but there was iron all around it which probably kept it safe in the ground all those years as other folks thought it was junk. Even that find though, is unusual. These days most older coins are fond in one of two two types of locations: 1) deep, hard to hear or locate coins at popular locations; 2) Obscure little known sites that have never been worked. Lets address these two types of locations individually:

1) Deep, hard to hear or locate coins at popular locations -
Any detectorist who has been spending much time with his unit knows that the local parks and schools have all been worked pretty hard. Even at sites that have been in use for many decades, very rarely will one find older coins near the surface. The near surface older coins have already been recovered and additional old coins don't get put out to replace them. However, some of the oldest coins have had a chance to sink deep into the soil. These may have been passed over by less powerful, older detectors. Because of its high gain, the MXT is capable of detecting deep coins that some other detectors have missed. Some detector operators even say deep coins have their own special sound. Keep a close eye out for weak, but repeatable sounds. Take a special interest in targets the MXT says are deep. This is especially true with repeatable targets where the MXT is unable to give a depth or ID indication. The MXT will detect targets that are deeper than the MXT's ID system will work (this is true for all discrimination type VLF detectors), so those repeatable but "no ID" targets may be the oldest, deepest ones. The ability to hear, recognize and find deeply buried coins is skill that can be developed with practice, and the MXT is a good detector for this use. You'll need to keep the gain as high as practical for that area, so see the comments above about keeping the MXT stable with increased gain.

2) Obscure little known sites that have never been worked -
Finding old coins at sites not worked before may not be too difficult (if the coins are shallow) but if deep, you may need your deep coin detection skills anyway. The tough part is locating the sites. Finding these sites usually requires research, or at least some good luck. Even with research, you will find that many sites which are seemingly forgotten have been already been worked, but occasionally one can find an old place that has not been worked before. In my opinion, you have to keep trying and be determined to locate these sites. Eventually your dedication will pay off. If you have access to private property older parts of the country, these can be especially rewarding, as they are less likely to have been worked by others. Examples might include Granda Joe's old farm, or that lot an old friend owns where they tore the church down so many years ago. Demolition sites can be productive too - where old buildings covered sites since before the first commercial metal detectors. There are a number of well written books with suggestions about finding these types of sites. Make use of friends and family contacts and do some research. If you are lucky enough to find them, these are the types of places where one can really score a big number of  valuable, older coins, because no one has been there before you.