You need a crossover, somewhere. If you don't mind running your speakers full range and your sub has a LP filter, then it will work. Personally one of the biggest benefits of a sub is taking the load, read high excursion and distortion, off the mains so I want a high pass on the mains. Life would be good if someone made a real nice adjustable 2nd order HP active filter. You could then blend with the sub just right.
Most full size "stereo" preamps have a sub out, but no HP filter to the mains. Just a mono out to the sub.
Why do you want to run XLR? Are you running a hundred feet to your amp? In a horribly electrical environment next to a degausing machine or arc light? Or is XLR the "thing" you read about?
I am old school and run analog crossover, though researching. Some desktop multi-function DAC/head/amp have a sub output. I do not know why we don't see more 3,4,6 or more channel DACs of high quality and the onboard DSP to do the crossovers. This is the domain of MiniDSP of course. I am just looking into host based crossovers. Maybe you can run a cheap DAC to the sub and good to the mains, but use a PC based app volume control. They actually make a USB volume control pot that will tell windows to set the vol. Lots of options. Quality, convivence, price is your choice. This is what I am thinking about for my desktop as I am not as picky as my stereo. On it, I have an X-KITZ crossover I will be putting back in when I add a mute relay. LR4 @ 70 analog. I had a cheap Rolls on it for a while. Thought about swapping the OP-Amps. In my last house I ran a DBX pro-sound crossover. I did run XLR as it was about 50 feet from pre to amps.