OK, as no one else has jumped in, I got curious, as I've been eyeing the A21 and compared specs and information online.
I suspect that the HCA-1500A is in the process of being replaced by the 'New Classic' line. I can't find one for sale new online anywhere. If you look at the specs of the nearest 'New Classic' amp, you'll see that it both costs more and is a bit derated compared to the HCA, which specs out very similar to the Halo (at twice the price) with just a bit less power, as you noted.
My 'gut feeling' is that if you've got a line on a new HCA1500A, you should snap it up like a hungry hound on a pork chop.