So you are saying it my be a better solution to have a pre-amp that has a left and right sub out?
Not saying which is better, just addressing practical issues of connectivity.
I'm simply saying.....
If you use 70Hz high-pass filters to block LF info from your main amp / main speakers ....
Then you cannot connect sub to main amp via speaker cables.
Reason: main amp is now only outputting 70Hz and above. It does not contain anything below 70Hz.....which is what the sub needs!
Sub needs to get LF info, so you need to connect it to preamp outputs via IC.
The sub will receive the full signal, and the user then sets sub's internal low-pass filter to control how high into the FR the sub plays.
The above applies to a traditional 2 channel listening system, not a HT receiver.
If your HT receiver has a single LFE output for sub (which most do), then run it into your sub's single LFE input (which most subs have). Typically, the listener sets the sub's crossover point inside the HT receiver. Once setup, the HT receiver outputs XX Hz and below into the sub.
XX = low-pass frequency that the listener designates