Line level if your reciever/pro has a sub output, speaker input if it doesnt is how I see it but I am no sub expert, just a user and a fan.
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Probably not that much different on a powered sub as long as your power amp is not clipping. If your power amp begins to clip becuase of the difficult load of your main speakers then you will be feeding distortion to your subwoofer as well.
Speaker level will probably require some form of attenuation on the signal fed to the buffer and filter in front of the subwoofer power amplifier while the line level will go straight to a buffer and then filter - that's all.
If you run an arrangement that takes the power amp speaker signals to the subwoofer which then filters and re-routes this "power signal" to your main speakers then you are potentially passing "power signals" through another passive crossover = VERY BAD IDEA = adding distortion and sucking power from your power amplifier for no justifiable reason.