What is the best way to hook up a subwoofer

I use a second set of speaker cables and go from the amp to the speaker inputs on the sub.
This way you get full range to your speakers and a clean signal to your speakers.

The answer will depend on what kinds of inputs the particular sub provides. What make and model is it?

Regards,
-- Al

I use the second set of Preamp RCA outputs to the subwoofer.

I own an ARC Ref 5 line stage and also have a self powered sub woofer. I mention this because we may share similar issues.

I just checked the ARCDB web site and note that the SP-9 has two main outputs. Al asks the correct questions. Also is the sub woofer self powered?? Does the sub permit you to adjust low frequency rolloff, phase and loudness. These adjustments are important in order to blend the sub with your main speakers. If the sub does not provide these types of adjustments, you may need a separate cross-over.

Also, very important point. What is the input impedance attributes of your amp and the sub? The ARCDB web site mentions that the SP-9: "[r]ecommended load [should be] 60K ohms and 100pF. (20K ohms minimum)." In other words, if the combined input impedance of your amp and sub are less than 20K ohms, you will overload the SP-9, which could cause sonic degradation.

I've posted a number of comments about this issue and provide the formula for computing combined impedance, as well as the somewhat unique solution that I employed.

After answering the questions raised by Al and me, I think the group can provide better advice.