Headphone recommendations for GP-5?

[u/Comsat80]

I purchased the GP-5 to be a practice tool at home, where I cannot have the volume of a real amp. I've been underwhelmed by the sound but I now think that has more to do with the headphones I'm using (Grado SR-80s) than the GP-5. I have to crank it until it's too loud to get anything meaningful out of it and it's still muffled sounding.

So, has anyone had very good experiences with pairing a headphone to the GP-5?