You want your subs to be the deepest, tightest and driest they can be, the last thing you want is an amp driving another amp, that’s just wrong and gives one note bass, that’s why line level drive the best for them.
The last thing you would want is to drive them from say a tube amps etc speaker output.
Probably the only amp driven output that could compete with line level, would come from an amp like a Krells or similar, that have absolutely no ? on bass performance.
Cheers George