Why don't you want to use the speaker level connections? It's the best way to do it hands down. It doesn't change the final impedance the amp is handling, it doesn't draw any power away from your main speakers, and it keeps the subwoofer in better synch with your main speakers because it will be seeing the EXACT same signal that your speakers see.
Just run one pair of speaker cables from the amp to the speakers, then another pair of speaker cables from the amp to the subwoofer. Don't daisy-chain the speakers to the sub's speaker level output because THAT will be detrimental to the sound.