T+A headphone out... greater than 50 ohms?

I'm a little confused and was hoping someone could help me out. I have a T+A Power Plant Mk II on the way and it states in the manual that only headphones "with an impedance of at least 50 Ω" should be used. Does that mean that easy driven headphones like Grado's with 32ohms are not recommended? Why would that be? I would think the easier headphones would be fine in any amp considering they can usually be powered by nothing but an iPod.

Does this mean I have to ditch my Grado's and buy something like Sennheiser HD 650's with 300 ohm impedance?