In short, it blows it out of the water.
The lowest note human ears can hear is 20Hz (although you can 'feel' lower frequencies)
The highest frequency human ears can hear is 20kHz, but most people can't hear above 15; and if you've ever heard a 15kHz test tone, I can almost gaurentee that you wouldn't want anything that high in your bass's sound.
The low B string of a 5 string resonates at (roughly) 32Hz; but most of the 'bass' in a bass guitar's signal occurs between 80 and 120Hz. Anything below that is more 'meat' than 'note'. Bottom E on a 4 string is 40Hz, but what you mainly hear when you pluck the string is 80Hz, the second harmonic. That said, if you whip everything below 50Hz out of your bass signal, you'll notice it, a lot.
A lot of what wireless does to your bass's signal isn't to do with the low frequencies though; in fact Sennheiser released a wireless system tailored specifically for bass a couple of years back, which had a roll off at 50Hz. With the exception of the new Line6 system, all wireless instrument packs (and microphones) compound, fold up, compress, squash and generally mangle your signal before it's sent across the airwaves, then do their best to unpack it at the end. The line6 system is digital, so doesn't do anything to your signal other than digitise it before transmitting it (it does this at higher than CD quality, so no worries there). This means that all of that tricky packing/unpacking malarkey just isn't necessary, and leaves you with a much better signal, and (IMO) a much better wireless system in general.
EDIT: well, walker beat me to it, and put it more concisely! But never mind, if you can be bothered to read my babble I'm sure there's something useful in there somewhere!
