One sub, a two channel system ,one HT reciever ??

Running the mains full range as Rel suggests, either works, or doesn't work...it has a lot to do with whats going on with the main speakers. The Kef's may work great...many smaller ported speakers may not, again....it depends.

Well said. I could not agree more.

FWIW: The safest approach is to NOT run the speakers full range. Although the best possible approach may be to run the main speakers full range with the sub just filling the bottom octave - this option is however restriced to equipment that is designed to work together and with mains that have woofers as big as a subwoofer to begin with. The same manufacturer and same design philosophy in the bass is required, IMHO - otherwise there is a fair risk of a "dog's breakfast" when just randomly pairing up gear...

It seems to me that my biwired KEF 104/2s are just a bit more transparent when the crossover is set to 80 Hz. They also sound fine full range without the sub, but the low pedal notes of a pipe organ, for example, are missing. The HGS-15 fills that in, and can provide that subtle more felt than heard experience when a big pipe is invoked. For HT, you really need the sub; for jazz and chamber music, my favorites, maybe not.

db

Rolling off their bass may enable them to play louder but when I have tried it I found the benefits mostly theoretical and the drawbacks obvious.

One of the obvious problems from removing copious amount of bass distortion is that you can now hear copious amounts of lower midrange distortion or you may hear the obvious dull compression of the midrange. Often these effects are hidden by bass distortion. Masking effects on the ear do work in a designers favor...a ported resonant and harmonically distorted bass with a lousy impulse response can hide a lot of other shortcomings. This is not well understood but as a consequence modest amounts of distortion are often regarded as pleasing - making the music flow and sound more natural by hiding the obvious midrange blemishes of a poor design. This may be one of the reasons it is so hard to match a subwoofer to a panel...since a good panel has very low midrange distortion it sounds worse when masked by the subwoofer (contrary to a normal speaker that benefits from added bass distortion: think your average car stereo which sounds pretty good despite all the distortion)

Bob , without getting personal I would like to ask if you have ever heard ANY of the equipment I have described. I am describing experience, you are mouthing platitudes The Spendor , for example , was designed by the BBC and used around the world by other national radio networks as monitor speakers. The Nelson Reeds were designed as monitor speakers for movie studios. I probably made a mistake posting on an AV Question as the performance parameters appear to quite different. I was a Quad dealer for over 20 years and of course they are famous for distorted sound. Note to self, no more AV postings.