Seems the PowerBloc 5 is a fairly dated unit. Two sites I came across showed it in manufacture from 2000-2001. Some specs and manual are published here:
https://www.hifiengine.com/manual_library/legacy/powerbloc-5.shtml
Back then Legacy was focused on Class AB and given the weight of the unit, it looks like this follows suit. Also, this was likely around the time that Legacy started branding their own amps, though they had been working very closely with Coda Technologies for years and continued to commission them to produce the Legacy amps. So this is likely a Coda-manufactured amp, but I could be wrong about that.
I used to be a Legacy Aeris owner, so the PowerBloc series was on my radar.