The 'box' more or less just processes the signal from the coil. A smaller coil covers less ground and weaves among trashy sites with only a slight depth penalty. A larger coil covers more ground and gains a few inches in the depth department, but is heavier and suffers discernment issues when both a good target and trash lie under the coil. A coil upgrade for the Ace would cost around $100. You have a several choices including OEM and aftermarket.
I think that the Ace series detectors in the stock configuration are pretty well suited to finding most coin size targets at the depth they are likely to be (there's considerable debate on whether coins really sink or not). If seeking larger targets, then, as nsdq suggested, a slow approach (discrimination off) will discover large chunks of metal (caches, etc.) at considerable depth. Also, once a certain depth is reached it really becomes more of an excavation than anything (two foot deep holes are actually quite labor-intensive and certainly not practical in public places)...
enjoy the hunt,
cc
