OP - I agree with your friend. When you properly set up a sub (preferrably two) it will not be noticeable until you turn it off. That is how things should, and can work out for you. I'd get one (or two) and an SPL meter and sub freq. tracks to place and dial it in so it does not overpower/ interfere with your mains, but enhances them. Once this is achieved (not difficult, just a little time consuming), you will likely never want to not have a sub again, almost regardless of how low your mains can go.
Sub Question
I tried to do a search, but could not drill down on this. Anyway, I have never owned a sub. I never really felt the need. However, I had a friend tell me that a sub does more than just enhance bass output and perhaps bass quality. He went on to say that having a sub takes pressure off the driver thus making the speaker do less work. Because of this, the speakers will sound better or have better clarity. Thoughts?