There are a few questions you are trying to ask.
First getting 5.0,
Most receivers, preamp will do this if you set the mains to "full" or "Large" and set the subwoofer to "OFF". The reciever will then route the LFE signal(Which is extra bass info, there is bass info on all the channels) to the Left and Right main speaker. Thus No LFE, thus 5.0
As speakers for channels grow there has been soom confusion of subs.
If any of your speakers cannot go down to at least 20Hz then you "may" need a subwoofer for it, but at that frequency you are actually feeling the sound. Because, reasonably priced speakers dont go that low (Their specs will say they go low, but dont trust that) we would really have to have a sub for each channel. Since this is not feasable, Most articles will encourage you to set the all the speakers to small and have the Sub handle all the bass info. This is because your preamp will use its internal crossover circuits and route all bass signals below the crossover to the LFE channel.
Your preamp bass cross over may be different from others. Mine is 100Hz.
Before connecting your fronts to sub we must understand the Following...
High Pass / Low Pass
High Pass - the signal as already been through your amplifier and is going to the speakers.
Low Pass - The signal is coming from your pream and the has not yet been amplified.
Why we first ran mains through the sub..
In the stereo world, most people will have a sub for each main. This assumes you have a sub that has variable cross over. This will allow someone the get full range sound (20Hz to 20Khz). I say this because if your main speaker bottoms out at 75 Hz then, setting the subs crossover circuit at 75Hz will send any frequency above 75Hz to your main speaker and the sub woofer will handle all frequencies below 75Hz.
(Let us the left channel as example). This is done by setting you preamp front speaker setting to "FULL". Then runing the line out of the left channel to the LOW PASS left channel IN on your sub. Then run a connector from you your LOW PASS Left channel OUT to the left channel input on your amp. Thus set the variable cross over to match your speaker capabilities.
Setting the crossover at 80Hz if for Thx certified speakers and systems, yours may be different or sound better ant different setting.
Since most speakers aren't full range we would need a sub for each channel, and one for the LFE.
Mine is 5.1
All speakers set to "small"
LFE to the Subwoofer.
I have an 18inch sub woofer and placed it 1/4 the length of the back wall at the back of the room. This give me a smooth bass.
Second sub woofer could be use to smooth out the bass response in the room.