There is a difference between the input signal (5.1/7.1, Atmos, etc.) and the output from the processor. Processors have what's called "integrated bass management." That means whether you have a subwoofer or not, you don't lose the LFE channel, it just gets routed elsewhere. With IBM the easiest thing to do is to let your processor route the bass. Running separate sub lines to full-range speakers won't add anything you can't do with IBM.
Consider adding separate subwoofers if you like to listen to movies at their full dynamic range. Let me explain a little. The main channels in a movie sound track are full range. The 0.1 channel(s) are Low Frequency Effects (emphasis on effects).
That is, L, C, R and surrounds are considered full range, with a supplemental LFE channel for your typical dinosaur step and howitzer shell landing. The typical HT speaker set up has only 2 full range speakers (L and R) with the remaining speakers as small satellites.
The problem without a sub is you route ALL 5 or 7 channels of bass to L and R. Obviously more with Atmos, but we digress. That may be OK with limited dynamic range and your average rom-com. It’s also true that usually we don’t have many channels at once playing loud bass, so that’s something to consider when weighing the pros and cons of adding a sub to a HT environment.
In summary, let your IBM do as much of the work for you as possible. Add a subwoofer if you feel you lack the lowest octaves and/or want to improve the dynamic range of your system and reduce the bass load on the L and R main speakers.