Dexed is a 6 operator virtual FM synth modeled closely on the Yamaha DX7, It also works as an editor for DX synths (though you'll need to check compatibility). It has been ported to iOS by George Rosar from the original code by Pascal Gauthier and the big news in this update is that he's added the programs from 20,000 memory cartridges - totaling over 640.000 patches. Which is of course bonkers, and should keep you busy:
We did some rough calculations and figured at 10 seconds each of previewing the sound, it would take you 74 days to listen to them all.
We're not sure even developer George Rosar has done that, though he did write some heavy duty duplicate checking algorithms to make sure they are all unique.
Dexed is available for iOS (ipad and iPhone) plus as an AUv3 format plugin. for a mere $3.99
That works out at $0.000006 per patch (yeah, we're all about the maths today)