Why the fascination with subwoofers?

Thanks @mapman 
I am not necessarily looking for affordable but rather something that would be absolutely transparent since I would be high passing the entire signal above 40 or 50 Hz that is going to my main speakers.  That is one reason I have been curious about the Vandersteen model since I suspect it is about as transparent as these things get and plus it is simple, and another plus is that it is balanced, which is how the cabling is in my system.  I have thought about the Marchand units and they do make a balanced passive unit with poly caps and "high quality" inductors, at a reasonable price but I have not pulled the trigger since my roll-off/hand-off at around 40 Hz seems to be working and my amps are powerful enough that inserting the crossover might be creating issues to solve an issue that doesn't exist.  Maybe I should give one a try just to see. 

@akg_ca 

The one big problem with all of this is that you need a good crossover to roll off the deep bass in your system and achieve all of these benefits...”

So, I gotta ask, where do I get that crossover?
Vandersteen makes their M7-HP, which looks like a high quality unit (pricy) and I would be interested but unfortunately the 100 Hz high pass is higher than the (40-50 Hz range) frequency that I would want.  Also, based on the images of their unit, this seems to be a bit more complicated than an in-line capacitor so maybe not achievable as a DIY project.

Back to the OPs question, I would look at one sub for an HT set-up and two or more for music.  By using mains with excellent sounding bass that is limited to a cut-off at about 40Hz, and then rolling in subs for their greater control at the lower frequencies below 40 Hz, I have eliminated low frequency room interactions and greatly increased bass drive. 

@gdnrbob 

I am glad you are considering the Vandy subs.

Hi Bob, you responded to a second post that wasn't clear on its own.
I am not considering Vandy subs but rather I considered the Vandy M7-HP passive crossover, which costs about $2,500.  I probably would have already tried it except my main speakers do great bass (twin 9-inch woofers in a sealed, i.e., acoustic suspension, enclosure) down to about 40 Hz, so I really need to cross over lower than the fixed 100 Hz design of the Vandersteen passive crossover.  Currently, I am using twin Aerial SW-12 subs which sound great and utilize a remote control that can be used to easily adjust the output level for different recordings. 

I am considering adding a third SW-12 sub to the existing two, which are operated as single stereo channels and are placed in the front corners outside of my main speakers.  I will place the third sub along the rear wall or in one of the rear corners outputting a summed bass signal.  I cross them over between 40-45 Hz and keep my baseline output level (0 position) at a low'ish level to just fill in the lowest frequencies rather than to "shake the building."  IMO, if subs sound bad, this is where most sub users have made the mistake - the level is too high. I sold bass reflex (i.e., ported) speakers that went lower but excited room vibrations on about half of the recordings I listen to.  Adding one sub did not correct that situation but switching to speakers only going down to about 40 Hz and adding two subs absolutely corrected it.  Bass is now solid and controlled and rarely interacts with the room.

If I were starting over, and didn't already own the Aerial subs, I would look very hard at Duke's Audiokinesis Swarm Subwoofer System (four subs) as a reasonably priced alternative to obtain great bass.

Do you have to play a mono (i.e., summed) signal from all the subs or would there be value to trying the front subs in stereo and the rear subs or one rear sub in mono to better deal with standing waves?