These readings could be caused by sulfated batteries or just plain undercharged batteries. They hydrometer should not lie if it is accurate and compensated for temperature. I only have experience with my single 130 Ah 31 where I took close readings with a hydrometer and clamp on ammeter. I would not trust any current readings on a charging source itself unless verified by other tools that are known to be accurate.
I don't recall seeing in this thread the length or thickness of the cables from your Powermax to the batteries. If the cables are too thin and too long then the PM will see 14.6 and shut down bulk mode, way before the batteries actually climb that high. When you put your other charging source clamps directly on the batteries then there is less voltage drop to deal with and it pumps all it can into the still thirsty batteries and more than the PM was capable of doing.
Check voltage at PM output and at battery terminals themselves when charging. Do not trust any voltmeter placed anywhere else along the circuit. Lots of underperforming charging systems out there due to too long and too thin of cabling between battery and charging source.
Others with GC batteries should chime in and state the amps required to take a near fully charged pair of GC's, to 15.5x or 16 volts. My 31 takes 6.2 initial amps to reach 16v after holding at 15.3v for an hour taking 1.5 amps or so to be held there. As the Sg rises the amps required to hold 16v taper to around 4 amps for this 130 amp hour 12v battery.
I'd treat your unknown GC batteries like Trojans and use their ABSV recommendations until proven otherwise by various tools, number one being a hydrometer and an accurate voltmeter taking readings at both battery terminals and converter output terminal
