Hi J,
Now I'm not an expert and I'm sure there others that can help you more than me. I been hunting with my mxt for a little over a year now and I have hunted trashy parks as well. Two parks I hunt have pull tabs,bottle caps,and the pesky foil out the rear. When I first started hunting these parks I was so disappointed because I found it hard to discriminate the junk and then to get a good signal. The first advise I have is read the owners manual and then reread it. I have taken mine with me to have as a reference in the field if needed. Next start in the preset modes. I found that till I got use to the machine this helped me a great deal. I set my threshold to just barley audible and then the discrimination to just under pull tab or the number 5 (yes I know i didn't find allot of rings but I was trying to learn what the machine was telling me), then I set the gain to preset and I soon learned that for these parks preset was to hot and I had to back it off to around 6 or 7. Remember also that the trigger switch can be put in forward position and this will help give a broken signal or just a blank threshold sound on those junk targets. I also don't have the money for a different coil and I hunt with the standard coil all the time and I have learned something else, if I slow down in the real trashy areas( I mean the ones that give you 3 to 4 different signals in one sweep of the coil "pull tab, dime/penny,screw cap, etc") this really helps the mxt separate the signals better and I can pick out the good coin/ring signals better. When I say slow down I mean "S L O W" ,a snails pace, for me I just barely move the coil from left to right. Example- just yesterday I hunted a old ball field that was littered with trash and I got the p-tab, penny/dime, p-tab signal on the vdi. I was able to separate the signal by just barely moving the coil and in the middle of the tabs there was a dime. I also noticed that when I pinpointed the tabs gave a shallow signal and the dime was just a little deeper. I also used the relic mode allot when I started because you have 2 different tones and I thought that the discrimination seemed to be better for me. I also watch for a full bar on my machine(vdi), if it says quarter and full bar then it's a quarter, but if it says quarter half bar and the signal jumps around then it's usually a screw cap on my machine.
The best thing I can say is "Don't give up" on the mxt it's a great machine and in time you will learn what it's telling you. For me it's been a challenging year and it's been fun because just when I think I have it figured out I get a surprise.
Sorry for the long post but I hope this helps!!!!!!
HH, Dave