Chefp2010,
Well... there you go. Now it's clear for not only you, but all "Beginners and Audiophytes", including me.
The problem is that this is not by its nature simple problem. We are talking about a crossover here. Now we know if you take a set of drivers and buy an off the shelf crossover from parts Express the results will be awful. That is exactly what we are doing with a sub crossover. You have a high pass with a second order slope in the receiver output to the speakers and a fourth order low pass filter at the LFE out. Things are a little better than the parts Express example as the crossover point is variable. In another way they are worse, as the crossover region has the potential, and in fact usually does interact with room modes.
Now there is a mitigating factor. Everyone is familiar with the sound of the human voice. This makes an off the shelf parts Express crossover a real disaster, as the response aberration will be highly noticeable as everyone has a really good point of reference in the crossover region. However for music in the pop domain and most movie scenes there is really no comparable point of reference to the human speech disaster. So for practical purposes if sounds good and not offensive in the bottom end everybody is happy usually.
So the take home is that bass can sound good and not be accurate and if there is no point of reference whose to know?
The other issue is that abundant research has shown that what people perceive as bass is at a much higher frequency than they think, unless you are a highly trained critical listener. In fact the area where people are most sensitive to problems is well outside the sub range. The higher the frequency at which the problems occur the more people will object.
Things however break down when you are reproducing natural instruments naturally miked and recorded with no special effects. People who have a good point of reference for cellos, double basses, pianos, tympani, tubas, and above all good mechanical tracker pipe organs, are going to cry foul, and they do. They will find problems in this area aplenty as a general rule. That is why they seldom use subs. To get those instruments I mention to sound like being there is very difficult, and I believe almost impossible with a sub placed in a different location from the speakers. The crossover occurring in the range of the fundamental frequencies I have mentioned is an added problem.
I can assure you that speakers, subs and receiver does not solve the problem.
To cut to the chase my advice is if at all possible set the mains to large, and use the sub to gently just fill in the bottom end.
If your speakers are not robust enough for full range duty use the small setting and experiment with the crossover point. Bear in mind the crossover frequency for the sub is likely to be different for the mains. It is rare in any crossover situation for the electrical frequency of the crossover point to be the same for both drivers being crossed over. When you add electrical and acoustic slopes together, then you get the symmetrical crossover. This is hard to achieve without measurements.
This latter issue actually goes to the heart of the problem. The slopes of the electrical crossovers in the receiver are fixed, but all speakers to which the receiver is connected have all manor of different acoustic slopes with none identical.
So that is why in my reference system I have designed an integrated full range speaker system that does not require a sub.
When I do use subs I design the speaker and subs as an integrated package designed to work together.
Let me show you what I mean.
These speakers have four compartments. Below the tweeter is the bass/mid in one sealed cavity. Below that is the BSC driver in its own sealed cavity. In the box are two 9" sub drivers in isobarik configuration. It is a band pass sub, with one sealed and one vented cavity. The 3db points are 27 Hz and 90 Hz. Roll off is second order. The bass/mid and BSC speakers are designed with a 3db point of 90 Hz and since sealed roll off second order. So there is an acoustic crossover at 90 HZ.
Now interestingly, i was able to get the Marantz pre/pro to duplicate the electronic crossover I built for these speakers. Now band pass subs have a slightly peaked response. So the crossover is symmetrical by setting the high pass crossover to 60 Hz, so the woofer slopes are second order (12db per octave) acoustic from 90 to 60 Hz and then fourth order (24 db per octave) combined electrical and acoustic slopes below 60 Hz.
Now the sub rolls off nicely second order above 90 HZ and to prevent HF radiation from the port the LFE low pass was set to 120 Hz.
Since most speakers are ported Qb4 boxes, it makes it very difficult to get a nice crossover.
QB4 ported boxes, the speaker type most commonly in use today, roll off fourth order. when you add the receiver crossover the toll of is sixth order, (36 db per octave). Now the sub will roll off above crossover fourth order at 24 db per octave. Sometimes the room will bail you out, at other times it will work against you. On the other hand a sealed main speaker or properly designed TL would form a symmetrical combined acoustic and electrical crossover. That is why THX in their specs and set up recommend a sealed alignment for mains.
I know this is a complex topic, and it is no wonder people are confused and often achieve unsatisfactory results. However with current practice as usually applied an awful lot is left to chance. So expect results to vary.
