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

When you use the line level in and out you are using the crossover and rolling your main speakers off at the subs crossover point.

On some equipment this can be a limitation yes. However, I think there are preamps out there that allow you to have control over each speaker individually so the sub can be crossed over or not and the speakers can be crossed over of not.

I'd look into the Anthem D1 or D2 if you want a high quality HT preamp with plenty of flexibility and that could also serve as a source for a decent two channel system or a two channel + sub system.

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...

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)

I was a Quad dealer for over 20 years and of course they are famous for distorted sound.

Do I detect some tongue-in cheek?

I would add that anyone with a typical modern system where it is often difficult to hear dialogue on movies without jacking the sound way up (background sounds cloud diaglogue) needs to ask themselves what might be wrong with their midrange and what might be wrong with the "impressive" bass....one word "distortion".