Comparison of various subwoofers for 2 channel audio

I would drop SVS from your list unless you desire one note boom bass

That’s a misleading and inaccurate statement.  I have an SB2000 sub, and while certainly not SOTA it is absolutely not “one note boom bass.”  Further, there’s a member here who had both the highly-regarded Rhythmik F12 and SVS SB3000 in his 2-channel system and found them to be very close in performance to the point of being almost indifferent between the two, which certainly would not be the case if the SVS sub was that bad.  Personally I’d take Rhythmik over SVS albeit at a higher price, but categorizing SVS subs as “one note boom bass” is an over-the-top and ridiculous statement.  

Of the ones you mentioned, I'd recommend Rythmik.  Adding another pair of decent quality subs will help even out nulls and peaks in your room and give you more headroom (the ability to reproduce those low notes with little strain on the sub) when there is deep bass present in the music.  More subs can also increase the sense of "realism", that you're listening to live music, versus a recording.

The comment that SVS subs are only capable of "one note boom bass" is inane, or at best, uninformed.  I have a SVS SB12-NSD sub in my computer room, where I'm currently listening.  It's a small room and it's paired with small monitor speakers (Harbeth P3ESR) and quite capable of filling in the lower register that the Harbeths simply can't produce. 

There is no "boominess" or "one-note" character because it's the right sub for the size of the room and the speakers and is properly integrated.  It's very easy to distinguish the tone of bass notes.  The combination of the small monitors and the single sub provide a very satisfying experience and the feel of much larger speakers without overwhelming the room.

In general, internet direct subwoofer manufacturers are going to provide the best value.  The brands that dealers carry have huge markups for advertising and dealer margin.

My main system, which is in a much larger room includes two Rythmik F25 subs (dual 15" with 800 watts each) and two SVS SB13 Ultra subs (single 13.5" with 1000 watts each). 

The levels on all four subs are barely on, and for the majority of the music I listen to, you wouldn't know they are there, but they come alive when there is a lot of bass content in the music.  Not only that, but they create an immersive listening experience that is more like what you would experience listening to live music.  My main speakers are KEF Reference 1, which have surprisingly good bass for their size, but can't possibly reproduce bass that would be in scale with the music in that size of a room.

Again, right size and amount of sub for the room (possibly overkill), and integrated so that they blend well and complement the main speakers.

Rythmik F12G owner here.  I don't own electrostatics, but have Magnepan MMGs.  The F12G is very fast and musical sub that has a very good array of controls to match your listening preferences, whatever they may be.  Can't go wrong with a Rythmik, IMO.  It marries very well with my Maggies.  I cross at 80 after much experimentation with 60 (right around where the MMGs drop off) and 90.  A 90 Hz crossover made the Maggies sound too thin, 60 was a bit too congested-sounding.  80 was the sweet spot that really crystallized the Maggies and tightened up the bass presentation.

I run the Audiokinesis distributed bass array. I found this article, recommended by Duke LeJeune, to be helpful for placement of multiple subs.  Tried to post a link but it kept including the entire 30 page pdf.  
 

So Duck Duck Go cut and paste:

"Subwoofers: Optimum Number
and Locations" by
Todd Welti 

@carlsbad2 wrote:

One thing i've noticed with subwoofers.  there are two types of listeners:  those adding subs just to make the bass natural--the way it was recorded.  And then there are those who want more bass than is recorded on the track.  I'm not criticizing anyone but these discussions go sideways as the two different crowds have 2 different answers.  It is good to define which you are.

Due to the nature of bass distribution acoustically in domestic environments I'm guessing quite a few are missing out on what naturally present bass can really sound and feel like, as an uneven and less clean reproduction of the lower octaves is typically gained back to not be felt too conspicuous one way or the other in the overall mix. Making matters worse in the context of subs is poor integration with the mains that will only exacerbate the issue of lowered gain. That being the case there's often a lack of proper foundation in music playback without people really knowing about it, because the issues seem to have been dialed back - but with them the sense of natural presence and fullness of bass as well. 

So, I'd argue that what many feel is a natural bass as found in the recording is really not a true indication of such, but rather a more or less anemic version of it. Don't get me wrong; subs dialed too loud with frequency irregularities/boominess, feeling like they're making an effort, marred by overhang and not properly integrated are a nuisance of which one is better off without. However, while the former scenario is less intrusive it's also, potentially, not enough of a presence to be felt truly natural as heard in live music - acoustically or amplified. 

The lowest octaves require the most energy to be felt "linear," clean and natural, and for that one needs much more displacement that may at first be anticipated, in addition to efficiency and/or a lot of power. Seeing subs of typically smaller size than the main speakers, even significantly so, is the first indication of something gone wrong here, but try and get that through to audiophiles who'll mostly laugh at the sight of large and/or stacked sub towers per channel claiming that it's way "overkill." What it really is however is exactly what's needed: "overkill" is simply common sense where the lowest octaves goes, as this is a way to ensure proper energy, ease and physicality of reproduction. 

I'm definitely an advocate of natural bass, but for that there's no way around the adherence to physics with all that implies, as well of course overall integration.