What's the difference between Proceed's amplifiers

The HPA2/3 are the more recent models, circa 1998.
The Amp 2/3 series dates to 1994.

John

Is there a significant difference in sound quality?

I have a couple of Amp-2s and an Amp-3. I think the HP series are larger and have greater power. I have read that the HA series doesn't have quite the same sound, but I don't know that. The Amp-2 and Amp-3 are fine amps, with an excellent power band, and balanced inputs. Each channel has its own large tyroidal power transformer.

db

There is a difference, how significant it would be is up to your ears/wallet to determine.

Why the curiousity about these particular amps? You do know that the Proceed line no longer exists don't you? Do you also know that Madrigal no longer owns/markets Proceed/Mark Levinson components? Harmon International owns them now. FWIW, I have heard that service can be an issue, are you aware of this?
I would think that unless someone was giving them away, I might be looking for another amp. There are many good ones out there, even on the used market.

I recently sent my Levinson amp in for service. It was turned around in about 3 weeks. I know that there was a time where service was an issue due to the transfer of ownership.

But in my experience - Harman did an excellent job servicing my old 336 amp (which sounds better than ever mind you). I wouldn't hesitate to buy a Proceed - I think they are a great value (especially the HPA series).

Just my 2 cents.