I think you will find the same results from both options you have. Most guys with two channel use the speaker connection because a lot of two channel setups lack the connections needed.
To take it to the next level adding and active crossover between your pre and amp (pre amp to sub, sub to amp) is best but more expensive and requires extra interconnects.
To take it to the next level adding and active crossover between your pre and amp (pre amp to sub, sub to amp) is best but more expensive and requires extra interconnects.