My probably erroneous inference about the vintage RS-1 sounding closer to the HP-2 was just a wild guess since I've never listened to any vintage RS-1.
My comment was referred to the RS-1 with buttons and black headband, the only model I've listened to, not to the HP-2. I'm using these with flats, I didn't like the sound with bowls, kind of a hole in the midrange using them. With flats I find them surprisingly neutral for Grado phones, quite warm, almost relaxed and kind of less resolutive, but still very "pratty" carrying a good deal of the fun factor of other Grado models. I like these cans, they lack the airiness that suits classical music and are too in front sounding, but I could live with them as my only phones without much trouble.
Rgrds