Subwoofer for Magnepan 20.1s Yes/No/Why?

I have a pair of magnepan 20.1 speaker and have been building a great system over the past few years- pass labs x250.5 amp and xono phonostage, audio research ref 3 preamp, vpi reference super scoutmaster turntable/10.5 tonearm with dynavector xv1s cartridge. I am not a bass freak, but on some recordings, I sometimes wonder if it would be worth getting a pair of subwoofers would give me that extra bottom octave (ie like the nola thunderbolts that Harry Pearson uses) or if that would throw off the coherence of their natural sound? What do you guys think?
Have you measured your in room frequency response? Whether or not you need a sub really depends on what your measured output is in the bass area. Your room will determine whether your drop off at 80hz, 60hz, 40hz or 20hz.

I have full range speakers, but my room kills things at 80hz, save for a giant 35hz peak. The only thing that works in my room, is to high pass the speakers to avoid the 35hz peak, and to place a sub behind the listening chair, and then EQ the sub.

Your speakers are apparently -3db at 25hz, but again, it's all about what you get in room.

You have nothing to worry about with coherence, etc, but successfully integrating a sub pretty much requires being able to take RTA measurements.
I found the SWARM by Audio Kinesis (Duke LeJeune) to work very well in evening out bass modes and giving me some low slam without mucking up the bass. From his website:

"The Bass System

The main obstacles to natural-sounding bass reproduction are the inevitable room interactions - which impose large peaks and dips on the bass response. By using multiple subs spread asymmetrically around the room, each sub will produce a unique peak-and-dip pattern at the listening position. The combined average of these unique peak-and-dip patterns is much smoother than any one of them would be, resulting in more natural-sounding bass with excellent pitch definition. The Planetarium bass system also avoids the subjective slowness of most subwoofer systems by taking room gain into account. Typical in-room extension is to 20 Hz, but the main advantage of this approach is bass quality, not quantity.

Planetarium Alpha and Beta modules available separately; bass system sold separately as "the Swarm"."
Hi Powderdocter, I have had my MG-20's for over 15 years, along with hearing numerous 20's and 20.1's setup in many different systems. Before you spend money on subwoofers I would recommend at least to possible changes that would dramatically effect the performance of your 20.1's to a much higher level:

1) I have never heard 20's or 20.1's really perform up to their ultimate ability without biamping with an external x-cross over. The whole speaker is effected, however the bass extension/power on the bottom end also really becomes much better.

2) Read my GON review of the Mye Sound stands for all the details regarding the astounding impact on the overall sonic performance of my 20's, along with the best bottom end kick I have ever heard from my speakers.
Awesome system! I have 3.6s also with Pass amplification. I used an awesome woofer (Hsu top of the line) and profiled with Rives and RS meter. One thing I found is that the speakers took a *long* time to break in. Almost a year and the woofer panels were still changing their response curve. I think (hope!) they are broken in now, and have almost completely flat response down to 50 Hz. And you should be down another 10 - 15 with the 20.1s.

Okay - so I've got a woofer that can do 16 Hz bass at 120 db and the 3.6s... I find that I'm turning it off more than on. First of all, there is very little music with sound down there - and I have looked specifically for 'fun' bass tracks. I have Mickey Heart Davos, I have the CD that came with the Hsu and has 16 Hz pipe organs, I've got some other organ stuff, other new-age CDs with some low bass here and there... But the tracks that really are truly awesome music and are bass oriented aren't 25 Hz bass oriented. They are 80 Hz, 60 Hz, 120 Hz, etc. bass oriented. Listen to Dead Can Dance, Serpants Egg, track 5 or 6 or so - you'll know the one, that *seriously* moves, and I can't tell a difference listening to it with woofer on or off. And I'm listening at 93 db or higher.

A few months ago when the panels were still breaking in I could hear the difference, but now it is *very* rare that a track has actual information that low... Still glad I have the Hsu, and I can use it for movies and elevate the whole bottom end a bit (cheat and move the crossover up a bit, sssshhhh, its just a movie), but for critical listening its of moderate assistance.

Oh - their latest woofer is really really fast compared to previous models, so could mate well with panel speakers. And they are much smaller, you can put one right behind each speaker on the ground. Some are buying 3 and doing wierd things based on room physics, apparently if you put one near the ceiling something good happens... Beyond my knowledge.

But whatever system it is, you'll be crossing over at like 28 Hz or something. Have you profile your current response?