High-level power amp output hookup to a sub being superior to line-level hookup is a debatable contention. Brian Ding, designer and maker of the Rythmik Audio subs, provides both on his models which contain either the A370 or H600 (non-XLR version only) plate amp. It is his opinion that high-level hookup is preferable.
Why would anyone want to add the distortion all power amps produce to the signal sent to the sub? People say a sub hooked up high-level is more apt to sound like the speaker hooked up to the same amp, which is to admit that the power amp is not transparent. The better the amp, the more the high-level hookup will sound like the line-level hookup, right?