You're almost certainly referring to the midbass. Here, take a look at this:
Notice how they all have a hump around 100Hz and in the case of the rs1 (ostenibly a premium piece) the subbass absolutely vanishes. Keep in mind that headphones with only a slightly boosted bass down in the low frequencies will sound thin, but what we're seeing here is NO low frequencies. In the case of the 325 and rs1 everything under 50Hz is actually held back, and even the PS500 is yanked down by 30. I do feel the need mention, again, that bass just ON that 0 line would be thin and weak, let alone the cataclysmic drop we're seeing between 100 and 10 (which makes the subbass sound even more feeble).
I really want to like Grado headphones. I think they're gorgeous headphones, they feel amazing in the hands, I think the way they're made kicks ass. I just cannot imagine listening to the dang things for like 95% of music.