GO for Citadel 1.5s! They are amazing even tho my mains have active subs and are still continuing to get better.
HPA3 = High failure rates and no aid from paulie walnuts sterngold in caSE of issues.. I got raked over the coals and never got my amp5 fixed. the HPA3 is of the same dna.
Harman = Anti consumer when it comes to repair and from the schematics I have seen there were revisions. and known issues...
Cit 1.5 or prometheus would be my suggestion. (went from an amp5 to monoblocks)