Will a subwoofer add depth and clarity to my system, or just bass?

Hello again Duke,

     I'm very grateful for your many contributions on this thread sharing your very relevant knowledge and experience concerning how to attain very good bass response in our home music and ht rooms.
     You've mentioned 'real world' examples and it made me think that's exactly what I consider myself to be.  
     So, I thought I'd give some details about my speakers, room, how my 4 Debra subs are positioned in my room and the results just as a real world example for illustration purposes in case it might help.  
     My combo 2-ch music and ht system is in my 23 x16 x 8 ft. living room. I have a large hdtv centered on the front 16' wall with a Magnepan CC3 center ch spkr wall mounted just above the tv and a pair of Magnepan 2.7QR speakers straddling the tv, about 8'apart and 3-4' away from the front wall.  Here are the important specs on the 2.7QRs:

6' x 2' 3-way planar-magnetic panels
QR tweeter : 1½″×56″
midrange : 98in²
woofer : 620in² 
34Hz-26kHz ±3dB 
87dB/2.83v/m @500Hz 
Low-Pass 12dB/octave @ 650Hz

Band-Pass 6dB/octave @ 500Hz-950Hz

High-Pass 6dB/octave @ 950Hz

bass inverted

     2 of the AK Debra bass system's subs are positioned along the front 16' wall, one about 2' away from the left-front corner and the other about 2' away from the right-front corner with both behind my main spkrs and the 10" drivers facing the front wall and less than 2" away from it.

     The remaining 2 subs are placed along the 23' side walls, one on each wall about 2' away from each rear corner.

     All 4 subs are run in mono and powered by a single class AB 1K w Dayton with the cutoff frequency set at 40Hz.  I experimented with the phase control, slowly adjusting it from '0' to '180' and back again numerous times while listening for subtle improvements in the bass response. I failed to notice any changes so I set this control to '0' (in phase) and proceeded to the next setup step. 

     Once all 4 subs are setup, the instructions call for sequentially reversing the polarity on each sub to determine if the in-room bass response is perceived as improved with the polarity reversed on one specific sub of the 4.  If I recall correctly, my friend helping me and I were both tired at this point and we both couldn't imagine exactly how the bass could be improved, so we never completed this step on setup day or since.  It's been 3 years now and I still can't imagine how the bass in my system could be improved.  Since everyone who's ever listened to my system, including my friend and I, believes the bass is sota,  I'm thinking there's no good reason to mess with it now and possibly jinx it. 
     My current opinion is that the DBA system is the best method to achieve sota bass response in any room and it will be able to integrate seamlessly with any pair of main speakers.  I know I'll never need to buy another bass system again and I can use any pair of speakers I want with it in the future.  The DBA concept honestly does perform that well and it costs about the same as 2 high quality subs.  I also believe this system has a high WAF factor in my medium sized room.

Tim

Umm ...

To OPTIMALLY separate the (low bass) subwoofer passband from the (mid-bass) main woofer passband, it's necessary to use a full 4th order (24dB/octave) active Linkwitz-Riley crossover.

The electro-acoustical sum is what matters, and it's more than just what you remove, it's how the subwoofer integrates with the rest of the system.

This is why THX used a sealed 80 Hz limited satellite as the standard.  Add 2nd order high pass filter, and voila, you have a total 24dB/octave roll off which would integrate nicely with a sub using a 4th order electrical low pass filter.

Honestly, only speaker makers and active room correction software does this anywhere near optimally, which is why I'm wary of recommending subs at all. It's not the tech is bad, it's the complications in the setup that I've so often heard as poor.

Best,
E

I've chosen a 7th order (42dB/octave) Linkwitz-Riley low-/high-pass at 80Hz for my tapped horn subs and all-horn mains via a Xilica XP-3060 digital XO - it's simply what sounds the best. 

Previously I ran my mains full-range with a single SVS SB16-Ultra low-passed at 61Hz 24dB/octave (via its own build-in DSP), and while this pairing integrated very well my new setup with two tapped horns (powered by a Crown Macro-Tech 1200) and managed by the Xilica unit, now high-passing the mains, even at this rather early optimization stage sans PEQ offers better, indeed seamless integration with the mains, while lending the presentation a sense of uninhibited "breath," ease and coherency not heard with the previous single SVS-installation. 

Aspects of this no doubt falls back on the inclusion of not one but two tapped horns, but running the mains full-range, by-passing the Xilica, with a lower ~60Hz low-pass of the TH's tells me all I need to know with regard to my preference; to my surprise even running the Xilica over the mains with a high-pass isn't detrimental in regards to transparency, but has a freeing effect on the sound with noticeably better energy-coherence and overall integration. If there is something robbed of nuance via the Xilica, I'm not hearing it (in indication of how good a digital XO it really is), masked perhaps (perhaps not) by the other qualities it brings to the table with the high-passing of the mains that only adds to the enjoyment and insight of the sound. 

In not trying to make a case with the above as a generalization of what works the best in every situation (certainly, it depends), but simply to highlight that the typically non-audiophile approach of steep filtered, high-passing of the mains in the 80-100Hz region can work extremely well - with a pair (not four) of symmetrically placed tapped horns, no less, and in stereo mode, God forbid.  

In my humble estimation, a subwoofer adds those bottom resonances and harmonics that otherwise would be missing. Any subwoofer should not be noticeable as being a source of the bass, meaning that it is subtle and definitely felt and heard, but not pronounced as a sound source.

A good subwoofer will support all the other frequencies . . . and they, in turn, will do the same to the lowest bass notes, which can be attested in a balanced system where the subwoofer is turned off . . . and then back on -- the difference is most palpable and fulfilling in its supplementing the sound of the main speakers.

A good example would be a good grand piano. The soundboard resonant the harmonics as do the strings and the cabinetry. What you don't want from you subwoofers are the standing waves that excite the room into a boominess, and a sort of one-note wonder that emphasize certain frequencies in the deep bass. The subwoofer should be capable of singing and playing a bass melody . . . not just a stangnent rumble or irritating thunder for the sake of room shaking bass. To me, it is the qualty of the bass, the solidarity of the sound that give a very satisfying feel and completeness to the overall presentation of the music.