its whats known as the grado "House sound" there are a few different "flavours" if you will though
I found the ps500 actually very good when i heard it.
The bigger grado's (as opposed to the baby's, 60/80, of which i own the 80's) tend to get a lot more refined. that sub bass says hi a bit more,
the treble gets more composed, but retains its forwardness. the midbass hump will always be there (kick drums sound lovely ;) )
they are never a true "hifi headphone" if you will. i don't mean to say however, that they are bad, they are in fact excellent.
I have headphones that are technically superior to my SR225i's
but that doesn't really matter all that much because nothing makes rock/metal so magic.
And so you would find the DT880 better, they are better headphones. for most things.. (besides they cost a lot more, and at this price range we haven't hit the wall that is diminishing returns yet)