If you get a chance, read the big headphone review that was conducted by a member here and a guy who works for headphones.com.
Go into the review sections on the Grado headphones. He basically said that there was little differentiation between the RS1 and the RS1i. Also, he said the RS2 was similar to the RS1 - but with small sound changes and smaller wooden cups. Also, the PS-500 was reviewed too - I believe. I read where the PS-500 has 3 of the back holes in the driver opened up which could be the factor adding more bass to that model. Additionally, I was quite surprised to hear how Grado had also opened some of the rear holes in the GS-1000 and PS-1000 headphones, too. That was a real eye opener for me. I guess they know how to tweak some more bass out of their headphones.
I've also read where the PS-500 and HF-2 have a somwhat similar signature - yet the differences are explained. The PS-500 did have some minor sound changes that will be described in the comparision.
Grated, this is just how another individual did the testing and wrote a complete article for grading and determining some of the best headphone values and great sound based on many makes and models.