I've got an Earmax,and have found it to be the equal of anything in it's price bracket,and well above.
Smooth,detailed,fast and clear.....the Pro has the advantage of higher power output,so will drive a wider range of phones.
As Nic suggests,also include the Sugden...and you could give the parasound Z-Amp a try also....quite cheap,but actually very good quality,and bags of power...your ears will give in before the amp does(I use a redundant one I had lying around for rock etc on the phones).
I'm sure Nic can give his opinions on the Slee,as can several others here,and the MF Xcans have their fans as well....I didnt like them personally,as I found them a bit lacking in detail compared to the Earmax.