The claim that an amp-level connection is better than a line-level connection for subs is an opinion not universally held. I have heard really excellent systems with subs that were connected via line level. Line level allows the possibility of a line-level crossover for mains and subs, a very flexible way to arrange things. Also, in most domestic rooms, the EQ provided by subs with PEQ apps will be of real value in getting smoother bass.
Another extremely valuable item on a sub is a continuously variable phase control. It is the only way of getting the smoothest (flattest) bass from a sub in a given position. The more overlap between sub and main, that more correct phase matters. I wouldn’t pay a lot of money for a sub without that feature. A +/- polarity switch is better than nothing, but no substitute for a proper phase control.
I agree that in most cases, two subs are better than one. By better, I mean capable of providing flatter and more natural bass response.