I think the most important lessons I have learned since acquiring quality audio gear for music a year ago are: Understanding that a sound is usually a combination of frequencies whether it be a single note or an entire orchestra; Crossover frequency plays a crucial roll in the quality of the sound reproduced by a 2.1 channel audio system, affecting stereo image and tone at the MLP greatly; Good mains that can reproduce quality LF to at least 40Hz (even lower is better) are necessary in order to reproduce good quality sound in a 2.1 channel system for Rock. The crossover range of the system is an octave below and an octave above the actual crossover point which should guide the audiophile in how to purchase and set up the components of a 2.1 system.
My speakers produce good quality bass with a LFR down to 33Hz. Taking into account the rule that our brains will 'fill in' the fundamental frequency for a given note if fed the proper natural harmonics for that particular fundamental, my system ideally (ignoring room response) should be able to 'apparently' produce sound down to 17Hz without a sub. But at what power? Certainly not at the power a dedicated subwoofer would produce. So, doubling the effective LFR of my speakers to find the lower octave of the crossover range, gives me the crossover point for my speakers of 66Hz. This crossover point will give me an upper octave range up to 132Hz for the crossover range. Wanting to keep the upper range of the crossover below 100 Hz, forces me to choose the lower crossover point of 60Hz, since 50Hz gives me a lower octave range down to 25Hz which is too far below the effective LFR of my speakers. Now my mains are producing sound to 33Hz (30 if they can) and my sub up to 120Hz (if using the LFE input where the crossover of the sub itself is disabled).
It turns out that crossover frequencies above and below 60Hz on my system do not give adequate results to my ears for 2.1 listening. The stereo image and tone (especially tone of drums) becomes unsatisfactory to my ears. Listening to 2 channel music (Direct Mode) gives me a reference on which to base my crossover experiments, which are done by ear from the MLP.
I notice that for my system, since I have speakers which can handle low frequencies effectively (they are very efficient), that crossovers other than 60Hz in 2.1 listening mode degrades the sound as compared to 2 channel listening. More than just the frequencies within the range of the sub and the sounds within the crossover range are affected by adding a sub and introducing a crossover point. The whole stereo image as well as tonal qualities of the various instruments are affected. It is apparent, at least to my ears, that introducing a subwoofer into the system alters the timbre of various instruments (probably as an effect of the crossover), even those out of the frequency range of the sub, maybe even more so than those that lie within the range of the sub. The 60Hz crossover giving the least amount of noticeable change on my system as compared to 2 channel listening.
At this time, I am undecided if adding a subwoofer to my system for music was a good idea. I need to keep on experimenting. Had I different main speakers which could not produce LF as well as my current ones, maybe my conclusion would be different. I certainly have heard systems which have benefited from adding a subwoofer to work in conjunction with the mains, which lacked adequate LF for Rock. I suppose the benefit of reduced stress and distortion on the amps and mains might be worth the addition of the sub, if nothing else. But I am not willing (yet) to raise the crossover point, to say 80Hz, to further benefit from this effect because I do not like the effect on the sound.
I hope those who are new to subwoofers and even audio gear may benefit from my experience....that's why I include this conclusion. My thanks goes out to all of those who have contributed positively to this thread. To you and everyone reading this thread, good listening to you!
2 songs that have been useful during my sub/system setup (Format = Band, Song):
A Perfect Circle, "Counting Bodies Like Sheep To The Rhythm Of The War Drums"
Sick Puppies, "Riptide"