With the 0404 there is a tendency for my HD580 to sound just a touch veiled. So I prefer the XCAN with the 580 because there is absolutely no veil with that combo.
You need low impedance and/or high sensitivity cans. It's not a very powerful amp. Most of my cans don't work. I've successfully used my Equation RP21s. I imagine Grados can be driven by them. I don't own any Grados so I don't know for sure.