I've had the HP-2s since they came out (I forget when I got them, maybe 1992?) - and I have the HPA-1 amp that was sold at the same time.
I also purchased some RS-1s a few years back, which have their pluses and minuses. The RS-1s have awesome impact, and sound rich and full. They are very easy to drive, and with the Grado amp they have tons of punch (amazing punch), and with an Earmax Pro they are lush and sweeten up a bit.
I love the RS-1s when they're on their game, but for me, they are way too uncomfortable for long term listening, and they also seem fairly colored - they definitely put their sonic signature on the sound.
Which brings me to the HP-2s. I actually have full pads on them, mainly for comfort. It rolls off the highs a bit, and since these cans are already a bit subdued, they are definitely dry and smooth set up like this. However, they have great detail, even with the pads, and are very comfortable and non-fatiguing.
Compared to the RS-1s, they don't have the same punch, or go as deep, and they aren't as bright and lively, but overall I prefer listening to them, and can listen longer. Right now I'm driving them with a Hornet, but I also drive them with the Grado amps. The Hornet is doing an amazing jobs with these hard to drive cans! As you would expect, they are a great match with the Grado HPA-1 amp, I'd do a head on comparison between that amp and the Hornet, but I'm enjoying listening too much to get scientific with my listening.
The RS-1s are exciting and do a great test drive. Most people who hear them at my place really love them. For me, though, they can be fatiguing, and the comfort factor is really bad. I'm glad to own them, and would never sell them, but they're not my main rig - actually that honor goes to a Stax 3030 system, but the HP-2s have been on my head longer than any other phones (since I've had them so long) and they see frequent use connected to my computer through one of the amps.