What percentage of audiophiles use a sub ?

It is not about the 'sub'. If your main speakers can get down to at least 25 hz, no need for sub. Otherwise, a sub is a must. Also, records in most cases needed to take out the bass as the cutting on the disk master was always a problem, groove width. Once cd's came on, and were in turn re mastered from the original tapes, it was very clear what had been dropped from the vinyl stuff. I've gone back on a number of records and compared the vintage cd, you will amazed what was dropped. All this being said, the integrating is still the biggest challenge to get it right.

I always hear "any system can sound better with a high-quality sub." That may be the case. I was never able to integrate one successfully. These days some of them such as JL Audio will self-optimize and are supposed to sound good.

The problem I always had is the level of low frequency information among recordings greatly varies. If the sub is set up to sound good with one recording, the next thing you notice is it's thumping you out of the room on the next and hearing nothing at all on the next.

If I were to integrate a sub into a system I would puposely integrate it to a monitor speaker such as my Aerial 5Bs that don't go that low. That way you can make use of the sub's capability without it walking on the speaker's bass and vice versa.

I think where I differ from most people re subs is most people want to replicate "accurately" what is in the recording. For me I just LOVE the stomach punch of good bass whether it was meant to be or not meant to be that way in the recording. That is not to say my music is All bass - it is not, but for me there has to be a healthy dose. I spend a lot of time getting my REL sub right and just can't live without it or I feel the music sounds too "thin" which sounds to me like listening on an iPod, in fact without the sub I wouldn't even be interested in hi-end audio. Again, I come from a music background which is all about bass - electronic music so my taste will differ from most on here.

Some rooms are difficult to make a "full range" speaker work. Without a dedicated listening room where acoustic treatments don't look out of place, sometimes you have to make due with what works. In my room I have had ultimately better results with stand-mount speakers w/sub(s).
I agree with Wireless that sometimes you have to adjust the sub to different recordings, usually just the gain. With music that is not bass heavy, your sub setup is right when you don't notice the sub unless you turn it off and then it seems like something is missing.

Although I have successfully integrated the Rel B-2 to Verity Fidelio Encore's (floor-standing 3-way) and Sonus Faber Auditor M's (2-way monitor), in my experience, subs integrate best with monitors. The best example I ever heard was the combination of Sonus Faber Guarneri Memento's and the Rel Stentor III. I love the Memento's and ended up buying them. As great as they are, the Stentor took them to another level, not just in the bass response as would be expected, but also and more importantly for me in terms of unexpected yet quite remarkable improvements in soundstaging/imaging.