In reply to:

If I set my fronts to “large” on my Denon AVR, the fronts handle the low frequencies. But how low? What is the new crossover?



There is no new crossover. Your main speakers will now receive the full frequency range of that recording.
In reply to:

How does the sub's crossover switch work? Let’s say I enable it and set the crossover to 60Hz, do all low frequencies get sent to the sub and the crossover setting acts like a “sub traffic cop” and only allows frequencies below 60Hz to be produced?



There's a slope involved, named because if you were to look at a plotted frequency response chart, it would "slope" down. It's a gradual replacement of the frequencies going to your sub or your mains. In other words, 60Hz is the center point, but your sub will get some of the stuff at 70Hz, and a little at 80Hz. Your Mains will still get some frequencies at 50, and maybe even some at 45Hz. (Don't take there frequencies literally; every reciever is different. What's important to know is that it's not a sudden cutoff.)
In reply to:

If I also enable the sub’s crossover and set it at 60Hz, what happens



If you're using the crossover on your receiver, you wouldn't use the crossover on your sub too.... you would be doubling up the effect. You want to turn the crossover on the sub off...or, if there's no off switch, turn the crossover frequency as high as it will go on the sub.
In reply to:

Does this make sense or am I out in left field?



Not at all! They're common questions!



::::::: No disrespect to Axiom, but my favorite woofer is my yellow lab :::::::