Ajinkya, the frequencies that would only be felt and not heard start from 20 hz onwards. Seriously speaking there are not too many woofers in the price range that Pradski or Anooj are looking for that would go so low.
And while it is important for the subwoofer to go lower than the fronts, it also depends on how it presents the low. Whether it is loose whump or tight slam. If the bass is tight, often times I can make do even with lack of depth in frequency. But that is a personal preference. Some people like echoing, decaying bass notes. There is one for everyone, I guess.
In Pradski's case, the subwoofer also has to be one that goes significantly low than 28hz which is how low his Diamond 9.6's go. And that, as you know, is pretty significantly low for a standmounter.