how many ways to hook up a subwoofer

If it's a powered sub there won't be any issues connecting it to the speaker output of the amp.  Connect the sub between the amp and the main speakers. Your amp won't have any additional stress because the sub's amplifier will be powering the sub. 

Contrary to popular belief, some engineers actually recommend using the speaker level input of the subs instead of XLR or RCA because then the audio going into the subs will match the voicing (sound signature) of your power amplifier and will be the same as the voicing going to your main speakers, instead of the voicing of your preamp going to a sub and the voicing of your power amp going to the main speakers.  RCA connections is not recommended due to their tendency to pick up noise caused by ingress and XLR is recommended for long runs to avoid noise issues, but regardless, speaker level inputs are the preferred method of many engineers, for the aforementioned reason. 

lonemountain:  You are talking about professional applications and not what's being discussed.  See 2:20 on link: The best way to connect a subwoofer (youtube.com)

 BTW: I have several systems, both HIFI and professional and I know what you mean.  They are all connected differently and speaking for myself, the difference is an audible improvement if connected as described in the video!  Try it!  This video will clarify my post.   Thanks for your input.