I suspect the impedance match sharing the output using line level might have some influence. between the two options. I have no doubt that speaker level might sound better, interested to know why as it cant be that running through two power amps keep the sonic signature. Unless ofcourse the HiFi power amplifier has a specific addition that is not a problem or issue with the sub power amplifier. Makes no sense, all HiFi amps are imperfect and all subwoofer amps are perfect ?
The drawback with speaker level is no option to EQ, unless this is integrated into the Sub. Also very difficult to measure and check crossover and phase as you can only play just the mains,or mains with sub, not just the sub. Making EQ more difficult. EQ ideally would be better managed in the digital domain or you are forced to go from A>D and D>A again for the sub. Not terrible but the better the D>A, A>D and electronics the better the sound....
I guess nothing is perfect
