I can only speak for Texas and our 12 years of having a dedicated marine division. Regardless of brand, a 1.33 ohm load isn't going to cut it here during July and August because its possible for the port locker to reach 140 degrees, and black gelcoat (if you have it) isn't doing you any favors. Add to that the fact that a Class AB can generate four times the heat at a similar current draw and you have decreased the lower efficiency further as you have dropped the impedance. You can fan cool the ampifier to death but its still generating an inordinate amount of heat at its core and that is unhealthy. Its kind of like treating high blood pressure with medication so that you still eat the wrong foods and never exercise. Sorry, but its wrong to only treat the symtom and not the root cause. If you have a 'hybrid' in Texas you better split it up into two amplifiers. Also, keep in mind that efficiency affects voltage and low voltage compounds thermal issues. So if you want to play big you better have serious battery reserves, a REAL shore charger and possibly an upgrade alternator.
As for the HD600/4, early production models were a little sensitive to the remote turn-on voltage falling off which happens out on the lake. JL Audio has a revision for this. Also, because its strictly regulated it pulls more current to offset a decline in voltage so that the output power doesn't drop. An unregulated amplifier will stay on longer and just produce less and less power as the voltage drops where as a strictly regulated amplifier must have a more aggressive protection circuit and will shut down at a higher voltage threshold.

David
Earmark Marine