Using speaker level to the sub then to the mains is an extra drain on the amp and the filters may keep things from blending well.
I like using line level better. This allows me to use all 17 watts to my mains and a good quality sub/sub amp combo. I honestly use a Janus Interphase 1/A sub amp/crossover. This allows for virtually transparent crossover at 100hz, loads of power to the sub(pair of nelson reeds) and never interferes with the amp or pre except for being in between them.
I like using line level better. This allows me to use all 17 watts to my mains and a good quality sub/sub amp combo. I honestly use a Janus Interphase 1/A sub amp/crossover. This allows for virtually transparent crossover at 100hz, loads of power to the sub(pair of nelson reeds) and never interferes with the amp or pre except for being in between them.