If I'm not mistaken, Mario uses very thick bullhide on his instruments. Which is not necessarily the best for the kind of vibration a strong, low sound requires. I'm not saying they ALL not work, it depends on the animal but bullhide is good for congas overall. Because the Iya is not hit with just the fingers (like a conga), bull on an Iya might have to be a bit thinner than you'd think in order to get the vibration. The Iya sound is very specific. I suggest calling L&H and getting a mule skin. Make sure to tell Mike it's for an Iya and he will know exactly what you need. My friend's Iya sounds ridiculous and has L&H skins on the cha and enu. Bigger sound than LP, Toca etc. I must add that my friend has a traditional set from cuba and they are made of a solid piece. I guess that might have something to do with it.
Regardless, we had tried other skins on it and it didn't sound as good as it did once we put the mule on it.
Another option is a good quality cowhide. Moperc sells very good cow and I recently put one on a friend's Iya and it sounded perfect. Even though the sound is much more mellow...
Note: the fardela is a must either way.
good luck