Here's a pretty good article that I find to be dead on...
http://forum.ecoustics.com/bbs/messages/34579/128214.html
A lot of guys think they have full range speakers, when in reality very few are (insert very expensive & very $$$$$) to achieve a true full range speaker. With the addition & proper integration of a great sub (or two) you will experience a new found musicality that a single pair of speakers just can't produce.
The issue is not bass extension, but roll off.
Now I agree there is a conflict here between HT/pop and classical.
Lets take the OPs Quads. The upper woofer has a second order roll off, the lower fourth order. This provides a bass extension and curve the designer intended.
Now lets put a typical receiver crossover in the mix set at 60 Hz say. The top driver now rolls off fourth order, the lower driver will start rolling off second order and convert to sixth order. I highly doubt this is what the designer had in mind. The sub on the other hand will roll off second order at 60 Hz. This is far from an ideal crossover splice. Allowing the sub to gently assist the lower drivers is likely to result in much more accurate bass reproduction. In fact in the area of crossover in the OPs situation the crossover will be symmetrical second order. The high pass crossover is acoustic, avoiding the problems of electrical crossover on the high pass end of the crossover.
Let us just consider for a moment what would happen if you had the concept of top extenders, and had receivers that could crossover to a driver a 4 KHz, to a powered HF unit of any manufacture at that frequency. Results would be awful.
The only reason you get away with this at all with subs, is because it is a region were the human ear is not as critical.
Ideally if this sub/satellite system is to become the norm, then mains should all be sealed or TL speakers with F3s between 60 and 80 Hz. Then with second order roll of to the mains roll off would be fourth order composite. Then you would have the low pass filter to the sub roll off fourth order. Then you would have a nice transition.
My argument that the high pass filter to the mains at least should be where possible acoustic, is not mine alone, but B & W favor this for their 800 series.
I just set up a sub to B & W 800Ds last week, and this approach gave the best listening result and the smoothest in room response.
Because of the highly aberrant signals sent via the LFE channel in Hollywood's whiz bang movies, you have the issue of physical damage to the drivers to consider, if not protected with crossover. However there are drivers up to the task.
If it is accurate music reproduction that the OP is interested in from his new Quads, then he should certainly give a listen to the system running the Quads full range and gentle use of the sub starting an octave above F3, which for the OPs set up would be 60 Hz.
Speaking from a speaker designers perspective, I would not intend or advise a speaker of mine having a crossover modifying the low end balance and LF taper of the speaker.
The OPs speakers are very much aimed at the classical market. They are definitely speakers that are at their best reproducing Mozart rather than Two Inch Nails.
