Wrong. Crossovers do not cut off hard the way you apparently think. It’s usually a 12dB per octave slope, designed to gently fade from one to the other, speakers to subwoofer. It is common to set your XO an octave above the F3 of your speakers. This allows for better integration.
An example using my gear: my towers are solid to 25hz. If I run them large, they would receive full signal information containing deep infrasonic too. An 18Hz tone at volume could easily overexcurse the woofer and that would be the end. I cross those speakers at 60hz. This protects the woofer, gives the subwoofer the work it was designed to do, and enhances the quality of the sound.
Another example with my standmounts, rated down to 34Hz and crossed at 80: when running a series of test tones through my system I was surprised to see my woofers moving when using tones in the low 20s! That signal is still there!!! The speaker is not bypassed.
The DefTech speakers are nowhere near capable of doing what your PB16 can do for you. Look at the review I posted earlier. The measured frequency response is nowhere near what DT advertises, and physics alone should explain it. Low bass reproduction requires large cabinets: much larger than the size of those towers, and orders of magnitude larger than the center channel. You have an absolute beast of a Sub which is actually way more capable of doing the job you insist those towers and center should do. Use it!
As far as Audyssey goes, I think you will have better success understanding it with the App. It does present its own challenges, but the control it gives you is a significant improvement over the on-board AVR version.