I use Super Eclipse III's with an ACI Force XL sub. The Coincidents are great speakers that already go pretty damn low without a sub. I do what you describe and run a line-out from my pre to the sub, and a second line out to my amp. This arrangement took quite a bit of fiddling to get to work seamlessly, but trial and error with some help from ACI and various posts here have helped get it right for me. It really is a subtle effect with speakers that go this low, but it works quite well once sorted out. It is a matter of getting the combination of the low-cut, phase and volume just right. Room position also is quite important, but in my case I was very limited. With only a single sub I wonder if my soundstaging abilities are compromised slightly. Ultimately the presence of the sub should not be readily apparent in 90% or more of what you are listening to (at least that's the case with the music I listen to). I only notice the presence of the sub at the very deepest of passages and I do prefer it this way. I do not like when it intrudes too much into the bass range of the speakers, or if the volume is set too high.
Good luck.