Quite the opposite. When done properly the subs and mains have appropriate high and low pass filtering, so their is no overlap. Running subs off a post fader aux allows you to send only the instruments to the subs that are appropriate such as bass guitar, kick drum, and perhaps keyboard. There is typically no reason to send vocal, sax, trumpet, overhead, etc, to the subs as it just mucks up the sound with garbage below the fundamentals of these sound sources. By keeping all unnecessary open mics out of the sub mix it tends to tighten the bass significantly.
As a caveat, the bass, kick, and keyboard levels must be appropriately balanced between the main mix and the sub. IMHO, the reason running the subs off an aux often fails is because people run the sub levels too high.
Another trick for tightening up the bass is to run high pass filters on pretty much every channel and adjust the frequency to the sound source. For example I typically high-pass male vocals at 180 hz or higher and female vocals at 250 hz or higher.
