The HPA and even Amp series were basically Mark Levinson amps rebadged with only minimal changes, many in the case. Proceed was started by Madrigal to test their performance in the digital domain before badging it as Mark Levinson, so they needed to round out the line....
I replaced an HPA 3 with an ML 433 and the difference was subtle, I still use my HPA 2 for the surround in my system. Great amp, fairly neutral, though YES you have heard of them failing, these amps are some 15 years old, capacitors fail and expecting more than 10 years from a piece of gear is demanding a lot. That said I have no plans to replace my HPA2, it is an awesome amp!
I replaced an HPA 3 with an ML 433 and the difference was subtle, I still use my HPA 2 for the surround in my system. Great amp, fairly neutral, though YES you have heard of them failing, these amps are some 15 years old, capacitors fail and expecting more than 10 years from a piece of gear is demanding a lot. That said I have no plans to replace my HPA2, it is an awesome amp!