Ron
This is not true. Subwoofer can be added and be beneficial to ANY system be it planar, cone or whatever. This is a fact. Many here are using subs and they know how this is done. It is a painful always tedious affair that takes time and requires measurements (and fine tuning with trained ears) for best results. Works beyond scrutiny have demonstrated that the best bass requires multiple Low Frequency sources scattered in the room in a regular pattern (Toole, Devantier, S. Olive) and irregular ( Geddes). The problem stems from the fact that the best position for bass is not the same as that for the rest of the spectrum and the characteristics, we , audiophiles hold dear ( Spaciousness, soundstaging, imaging, etc)... Moreover bass enclosure tend to require some volume, so there is a limit to how big you need your main enclosure to be, thus the solution to have separates enclosure to produce bass... The so called multiple columns of which the New YG TOL, Gen 1.x, the Pendragon, The VR-11 , 101 etc are good examples, but we do find other manufacturers providing their subwoofers for the same, Magico with their Q-Subs, Wilson with the Thor Hammer, etc... What we seem strangely unwilling to accept is that it is a business and the manufacturer want to sell their own ware rather than peddle that of others... It would thus be strange for Wilson to turn around and tell you it would be better to use JL Audio or Seaton with their Full Range! Or God forbid to use the Magico Q-subs with their XLF!!
Moreover the headroom and linearity in the bass that you are mentioning will come from
more subwoofers... and it can be integrated to the point of total disappearance of the subwoofers as sound sources. And it doesn't require that the subs be of the same brand. This point is the truth. THe perception and it is encouraged by the manufacturer and perhaps helped by the audiophile psychology, is that subs and mains need to be of the same brand. We, likely feel comfortable with an All -MAgico or Wilson or Martin Logan "system". Truth to be told the manufacturer likely would have provided ancillary components to help the integration in the form of controller , crossover and in some cases DSP... but it could well be that those components from other manufacturer could provide better results.. JL Audio is a case in point. It could be said also that some subwoofers from other manufacturer handily outperform those from Famous Full Range manufacturers.. I could cite the stupendously good Seaton subwoofers, the JL Audio subwoofers, the Paradigm subwoofers. Those subwoofers are superlative performers and could teach a thing or two to brutally priced subwoofers . Our hobby is definitely not predicated on ROI: spending 10 times more more often than not result in a 5% improvement if that much and often a 0%!!. We are reluctant to admit it but we develop a certain orthodox thinking and some things/brands don't resonate with our audiophiles sensibilities so you rarely see people going for Seaton or Paradigm. JL Audio has found a way to be accepted and for good reasons...
Back to the matters at hand: headroom and linearity. These can be obtained with "grafted" subwoofers. The more subwoofers, the better in term of both linearity and headroom...Better than even 4-columns systems can manage And strangely in term of better perception of the sound elsewhere: The midrange opens up in a startling way when your bass is clean, even the treble comes in better focus and cleanliness. It is not easy thing to accomplish and (again!) requires the audiophile to think outside his/her comfort zone. There are some stupendously good subwoofers out there and they can help bring many a system to the 4-columns level at a much reduced cash outlay and often better objective and subjective results. I would be willing to bet that for the cash outlay of say One Wilson THor Hammer + Appropriate amp to drive it,that a Quatuor of JL Audio Fathom F212 or Seaton Submersive 18" would not provide much, much better, much more linear, results and multiple time the headroom... One could counter with dual Wilson Thor Hammer in which case it is more than likely that 4 (Other brands) subs well positioned and configured ( appropriate, EQ, Level and delay and not all in front people !!
) in a room will surpass the performance of the dual famous manufacturer subs ... subjectively as well.