I haven't read through the whole thread, so, my apologies if this point has already been made.
Years ago, I worked on integrating a Velodyne Optimum-8 sub with my NHT Classic Three's. Now, it's important to note that another persons measurements with the exact same gear, set up exactly the same, could yield markedly different results because the major factor in bass management is the room; and, frankly, my room sucks!
But, my point is, that subs are almost always going to impact other areas of the musical spectrum. As you can see from my measurement graph that I've attached (featuring multiple volume and phase settings), increasing the bass output below 60hz or so often times had the effect of sucking out the midbass in the 60-200hz range.
I finally decided that what my ears where telling me, that it sounded best without the sub, was the route to go. Again, in another room, perhaps it would have worked out better. As in everything audio related what works in one system may sound like crap in another.
What were your crossover frequencies and slope? NHT's were connected to sub so that the speaker pair were part of high pass, right? You used one sub, right, not two?
