1000W max at 1 ohm for a Orion 2100 HCCA, hmmmmmm.
65-70% efficiency is actually being very generous I think it's a bit lower than that.
I have seen a 2100 HCCA bench tested at over 1600W at 1 ohm mono, hell even an XTR-275 bridged down to 2 ohms mono (it's only 4 ohm stable mono) did 596W. I was getting like 700+ out of my XTR-2150 at 4 ohm mono (14.5V + 140A alternator, big 3).
The XTR-2250 which is essentially the high power version of the 2100 HCCA is known to pump out close to 2000W, they don't call it "The Beast" for nothing.
The NT-200 bridged down to 1 ohm mono did like 996W so there is no way a 2100 HCCA only mustered 1000W unless something was weak or wrong with it.
Keep in mind it took 14.4V and lots of amperage to get the output I listed above but they are certainly capable. A minimum of a 100A power supply like the one's used by Orion would be needed.
I would say the 2100 properly ventilated with ample voltage and current could handle 0.5 ohm stereo load daily.
I hate the way Class D amplifiers sound, stale and lifeless I tried an Orion XTR-1000 Pro and hated it, but did have a MMats D100 for a short time that I actually thought sounded good.