With all due respect to Jmcgrogan2, the fact that the amps are out of production should have no bearing on your decision to purchase them.
Buying these amps puts you in the same boat as ANY amp that is out of warranty. You will have to pay for repairs no matter who made it. Any technician worth their salt can repair an amp. The only caveat to this is if certain transistors become obsolete. The Proceed does not have this issue (at least for now) but it is just as possible for ANY amp by ANY manufacturer to get into that situation.
And, as others have noted, Harman still supports the product line with respect to repairs AND warranty (if it is still under warranty). That being said, the Proceed amps are VERY reliable and I have owned two AMP 3's and an AMP 5 with no issues.
Kudos to you for recognizing a bargain when you see one.
Now, to your original question, the BPA series were more of an evolutionary update to the AMP series (yes, there were electrical changes but nothing earth shaking different in the sonics). The HPA series were the higher-power versions 250w vs 150w and reportedly, improved sonics over the AMP series (less dry).