How do wireless subwoofers do this?

I have 2 of my 5 subs connected wirelessly. They don't sound any different than they did when I was using cables.

A download of the Syzygy application will give you a clear description of their DSP design. The 120 or 80Hz crossover settings your referring to sound like a surround receivers LF/Sub bass management settings.

If your electronics offer a separate pair of left and right RCA/line outputs they're what you'd use to connect to a wireless transmitter or directly to the subwoofer. 

Many powered subwoofers provide crossover adjustment as do a few preamplifiers. Since DSP can vary greatly crossover adjustment can range from a simple variable dial to a wide range of multi frequency bands of analog filters or digital parameters. 

This level of detailed crossover adjustment has been included on a few powered subwoofers and in the aftermarket for over a decade. To the best of my knowledge reviewers have only commented on the ease and improvement of a products auto equalization but little or nothing on the potential of a products manual adjustments. Its not a widely used tool but the effort and results can be quite satisfying.

Thanks to erik_squires whose been commenting on his methods.  

Both my subs are wireless but I have never bothered to try them wired so cannot compare.

However I went wireless for a different reason to most I am sure.
With my old wired sub I was getting some ground loop hum I could not eliminate.
The first wireless sub cured that and even though I have changed main equipment since which may have also fixed the ground loop hum I have never bothered to try wired again.

Besides as noted it does make placement and cable runs much less of a problem!

Sorry if this is a bit off topic but those of you running two subs, I assume you use the right and left channels from the second preamp output to drive the two subs. That being the case, if the preamp output impedance is 600 ohms (@ 1KHz) and the to outputs are in parallel, would the preamp still "see" 600 ohms from the main output as well as each output, left and right,  from the secondary output?  Just trying to understand how would a single (tube) preamp handle these three separate amplifiers without any negative impact on the bass performance due to possible impedance mismatch.