I dont own a MXT but i do own its bigger brother the DFX,i have a fair few coils for this machines including large coils upto 18'' but i must admit they rarely get used and then it would only be on say open pasture or ploughed and rolled farmland.These larger coils of course do go deeper than say a normal stock coil but mainly only on larger artifacts that the gain will be noticed,alot of folks fall into the trap of buying these larger coils thinking that they can be used on parks and instead of the stock coil for everyday use,this is far from the trueth infact the performance will be reduced.
Someone has mentioned about detecting a single coin at 36'' i am not doubting this comment but must admit that i have nver even gained anything like that depth from such a coil,even on the box that came with my 18'' Detech coil it basically mentioned that it is mainly for artifact hunting and would be totally useless on coins especially at depth and 36'' would almost be impossible,the rule of thumb when using larger coils is that the larger the target the deeper the coil will go and of course another boig problem with big coils is the possibility of multiple targets under the coil at the same time this of course can give null signals if one of them is iron.
Its a great talking point with all your detecing mates that i have the biggest coil going,but in reality it means nothing,these large coils have many downside ie weight,nulled signals,cost and also the proper location is vital to use them.The up side of using such large coils after using them for a fair few years is almost zero in my mind,i will admit i do own and use large Mono coils on my Pulse machine but we are talking about a specific use for such combinations and this is when a large coil does come into play and it would only take just one decent deep find that could pay for all my detecting equipment full stop and more.
Good luck with your large coil,i am not knocking them just tring to highlight a few things when buying these larger coils and the problems that can often be over seen.