G'day
Its a fact that some coils will work better on some detector models than they will do on others, what works well on a gpx5000 wont necessarily work well on an early model 4500 and so forth, but what will work well on the 5000 will also work well on the later version 4500, and this is also effected by where you are using the coil as the ground conditions from state to state can also have a part to play in it.
Every detector I have owned I follow the same procedure, I buy and test multiple coils before I settle on a few that I have found to be the most productive and also the easiest to use, you don't have to buy new coils to do this and one of the reasons I say this is because some of the early version coils that people no longer want to use because they are not the latest and greatest can be found to work extremely well on later machines of course providing they are in good working order, for instance through trial and error I have found that the early version Nuggetfinder SL coils, the light grey predecessors to the Advantage models work very well on the early 4500 where they are not as bright on the 5000 and later 4500.
I replaced the Advantage ones I had and normally used and found to be ok but not great ,with the NF SL coils, so I have the 10" round that is very hard coil to get but very hot on the 4500, plus the 14" round, 16" round and 20" round, the only other coils I have and use are the early model minelab supergoldsearch 8" mono and the minelab commander 11" mono, on occasion I use the nuggetfinder 12x8 advantage mono as I have not been able to lay my hands on a similar sized SL version to replace it with, last season bought the camo coiltek 9" flat wound mono to try out, but I a still undecided on that coil as I have not used it very much.
I was not overly impressed with the nuggetfinder evolution 12" mono in WA ground conditions I found it to be erratic and a bit dull compared with what other coils I had been using but you still have to try them out for yourself as also what might suit you may not suit others and vice versa, its a case of suck and see, but its worth the exercise if you want to do better on the gold, and also don't fall into the trap of just using a single coil all the time, changing coils for different conditions and ground types can make all the difference to your finds.
cheers
stayyerAU