I just recently bought a klipsch rsw 12, a beast of sub might I add. I went to klipsch website and noticed that it said the sub was compatible with 5.1 channel surround systems. So my question is when I hook this sub up via LFE input is it only decoding bass for 5 speakers and not the back 2 or what.

I have a 7.1 Onkyo 706, so I'm just wondering does it decode bass for the back 2.

Thanks in advance.

LFE is LFE regardless of how many other speakers.
If you set it up properly in your AVR you will get the frequencies below the x-over point you set along with the LFE whether you have 3, 5 or 7 speakers.

A sub is a sub when it comes to 5.1 vs 7.1. Just set up your bass management as necessary (IE" speaker size, crossover, phase, etc). Beyond that, your sub doesn't care if 2 speakers or 10.

lfe is lfe regardless of how many other speakers.

If you set it up properly in your avr you will get the frequencies below the x-over point you set along with the lfe whether you have 3, 5 or 7 speakers.

+1.