People who wear Grados are bright. People who don't are dull.
The tonal balance on a pair of Grados is skewed toward the high frequencies (HF). That's because their open-air design reduces resonance but lets bass leak out, leaving that imbalance. Unamped, the Grados sound a little lite. Guess what also sounds a little lite unamped? The HD800, the T1, the HD650, the K701, you name it. Closed cans are darker and give more thump but unless you pay for the best, you're going to have problems with dull treble and echoey resonance.
Because Grados are low-impedance cans (32 ohms compared to 64 ohms or 300 ohms or 600 ohms), it's common to run them unamped - and to judge them unamped against amped headphones from other firms. This is as unfair as downgrading the sound quality of a higher-impedance can because it just doesn't sound as loud (it takes more juice to get the same sound off a high-impedance can). If you amp the Grados with a high-quality amp, you will get performance you couldn't have imagined from the same cans unamped. I've had a number of cheap amps, but when I hooked my SR60s up to an M^3 with STEPS and a variable bass boost (as well as a crossfeed switch), the difference in sound was remarkable.