If a subwoofer can accept both Line Level In (LFE) and Speaker Level In and my integrated amp has 2 sets of Pre-out (one to connect power amp section and the another is a spare for future expansion / other application), should the subwoofer be connected through the line level or through the speaker level? Which one will be better?
If the outputs are pre amp only, and not a dedicated LFE output, you should try both connections. Running from the speaker level outs will allow you to use the subwoofer's high pass filter to remove the deepest bass signals from your main speakers. Sometimes people will prefer to take that load off the main speakers and sometimes people prefer to run their mains full range with the subwoofer only picking up the information the main speakers cannot successfully manage.
I have 2 x 10's 600watt (RMS) each with dual voice coils. They are rated at 4ohms per voice coil. My amp is 2400W Max @ 2ohms (1200W x 2). It can either be wired 300W x 2 @ 2 ohms (RMS), 600W x 1 @ 4 ohms (RMS), or 300W x 2 @ 4 ohms (RMS). How should I wire them to make the most power?