I owned a pair of Er-6s about 2 years ago. I had a problem with their lack of bass and somewhat tinniness and sold them to another head-fi member. I certainly saw the many benefits though and i know tons of people love those things.
As far as an amp goes. I have the absolute PERFECT amp in mind but its probably a bit too expensive. The Ray Samuels Tomahawk($295):
It is designed for IEM's(in ear monitors) like the ER6's. FWIW, the amp i have is the ray samuels hornet, which is almost identical cosmetically but was designed with full sized headphones in mind. For me, Ray Samuels blows other amp makers out of the water. The only other makers even in the same league are Tyll Hertsins of Headroom and Phil Larocco at larocco audio. Ray just finished making what looks like a piece of art and what is now probably the best consumer headphone amp in existence, the B-52($5,350):
I have tremendous respect for Ray's work and think it is the best in the business.