Well my speakers go down to 34hz, and I enjoy them, sometimes I wish there was more on the low end, but everything else is so good why risk it? I have NEVER heard a seamless integration of a sub, regardlees of size or location or quantity....etc. If musicality is your goal I don't think an 18" speaker has the ablity to be musical when being added to an existing speaker, but that's just what I have experienced. I suppose I am the closed minded purist who refuses to believe that adding one more "weakness" will improve the sound enough to justify its short comings. There are more problems with phase, cross-over points(never right!), dB peaks and valley's just too much to take on at once. And I have heard REL subs before and they do come the closest but nothing seems to work with my speakers and my ears, though I have been called crazy more then a few times ;)
~Tim
~Tim