Best place to put a single subwoofer?

Martinmobile, what is the rest of your system ?, amp/equipment, etc. My experience with MBLs have always been with Krell monos, and, a Crown Macro Reference. They love lots of power and lots of current, especially at a louder volume. I am not sure what spls you are trying to get, but you should use you Radio Shack meter and find out. Maybe you are taxing the MBL's to beyond their limit of staying clean and controlled. What are the panels you had built made of. They might be sucking out some of the bass punch and slam. Do you have dedicated outlets for your amps/equipment ? This all makes a huge impact (no pun intended) on what you are looking to achieve. I also want to add that sub woofer matching and set up is one of the hardest things I have encountered in all my years in this hobby. No offense meant in anything I have stated. MrD

MrD, no offense taken. At this point in time I am sponge and I am open to all input, suggestions, questions.
My system is as follows, (and is a mulligan stew of stuff):
Amps: B.A.T. Vk600se monoblocks (solid state) 600w/4ohm
Preamp: B.A.T. Vk51se (tubed)
Digital: Theta Digital Carmen 2 transport
Theta Digital DSpro Gen.5a dac
Analog: Linn Sondek LP12 w/Akito arm & Linn Adikt m.m.
PS Audio GCPH (solid state) Phono pre
F.M.: Magnum Dynalab MD102 tuner
Magnum Dynalab MD205 "signal sleuth" f.m. amp.
Speakers:MBL 101e Mk2's
Sub: JM Lab/Focal Utopia (Berrillium series)
Cables: All sorts of different shit. AudioQuest,Nordost,
Cardas,AcousticZen,Transparent. Some of it older
versions, but all of it either reference caliber
and/or top of the line in their respective brand.
(too many to list, too many to remember)

I have 5 dedicated 20amp circuits, 2 for the front end, 1 each for the B.A.T. amps., and 1 for the Subwoofer

The custom wall panels are 24"wide x 8feet tall x 3"thick (the frames that is). They are stuffed with "auralex-type" open cell foam that is 2" thick, then wrapped in in 1/2" thick batt insulation. The panels are then covered in a loose, open weave wool/polyester fabric. (Gotta look pretty for the wife. WAF is super important right?)

As for SPL's I am trying to "achieve"? Not particularly interested in any such maximum SPL's. Just want it CLEAN. And it is. Using my "hokey" little Radio Shack sound meter, it is entirely possible to reach a continuous 100db. NOT that I sit in the room and subject myself, or others to that for any length of time. But it will play loud as hell, as would be expected for MBL's. I have actually seen the spl meter hit 108 to 110db peaks. I know, friggin' crazy huh? (I don't know how entirely accurate a $35.00 Radio Shack meter is, but the sound pressure in the room is devastatingly "unbearable" at those extremes).
To reiterate my thoughts from earlier- it's really quite good. NO, actually it sounds KILLER. Just not KILLER enough, at least where bass is concerned.

Martinmobile, imo/ime, all seems in order, except for two things. Because of the omni pattern of the MBLs, much is being absorbed by your panels, although I do tend to like a "dead" wall behind the speakers. As an experiment, try removing some to see if the slam comes back, even though the brightness will return. Second, the level of your equipment deserves a second sub. Room coupling is always better balanced with a pair, ime. However, as I stated, getting the "right" subs to match the mains (coherence, speed, tone, etc.)is a bitch, and much harder as the system gets more "refined". This is why many people (Audiogon type) do not like subs. Much of the "slam" you are looking for is above the subs crossover point, so it might not be the sub issue. Are you running the MBLs full range, or rolling them off at the last octave ?(again, very tricky if you are). I am not one to think the BAT gear to be the most "ballsy", ime, but your amps are great none the less. Are your tubes (pre amp) experiencing microphonics ? I would love to listen to your system, as I am huge MBL fan (although I own horns, which is another thread altogether). Keep us posted. MrD

Hello Everybody. Hello MrD, Thanks for the feedback. I am not rolling off the last octave and I am in fact running the MBL's full range. I am not a big fan of routing the audio signal through Hi-pass filters in the subs crossover. I have found that to degrade the sound to the main speakers, in my experience, so I run my sub straight from my preamp, X'd-over at between 50 and 65hz. My MBL's run full range. (I did the same thing with the Martin Logans and the plethora of subs that I mated with them. Allways full range).
I agree about the running of stereo subs as I have indeed done that. Dual Velodynes, Dual Bagend Infrasub 18's, Dual Von Schweikerts, and even Dual Sunfire "True" Subwoofers-God forbid! (which by the way were absolute pieces of shit for music. I refer to them as "nuclear bombs in a small box"- No pitch definition, 1 note bass pieces of poop that they were!). Yes it is difficult to make a pair of stereo subs integrate properly, and does not take much to overdrive the hell out of the listening room.
I have given some thought to the fact that maybe I "over-damped" the wall with all those big panels I had made, and have considered taking a few of them down, like maybe 2 of them. Leave 3 up? But I have not done that yet. The one thing I am mindful of is how much that wall behind my MBL's reinforces the sound, making the overall musical presentation rather "bright" and "strident" sounding. Not a good thing IMHO. Been there, done that, didn't really like the effect. Room definately sounds better with a "dead" wall behind the speakers. But I should try taking a FEW of them down and see what the effect is. Good suggestion MrD.
As for the tubes in my preamp going "microphonic"?? I haven't a clue. How would I know? What is "microphonic" anyway? (showing my tube ignorance now. This B.A.T. preamp is the first AND only piece of tube gear that I've ever owned. Always had solid state my whole life.) I have heard the term "microphonic" used, but wouldn't know if something like it is occuring with my toys. Please enlighten me if you would?

Martinmobile, microphonics are the result of unwanted vibrations via acoustic (airborn) or mechanical feedback, usually re-amplified through the system. These can enter (or exist in) the preamp chassis and screw things up. IME tubes are more prone to this, although, it is everywhere and in everything. I believe heavily in "tube dampers", as I feel the sound is cleaner and tighter. BAT might have included them with the pre amp. They look like garden hose washers (as an example) and come in different shapes, sizes and colors. There are so many tweaks available to "tighten" up the sound. You are free to email me and I can discuss some of these with you. Taking panels off the speaker wall would be an experiment to see if " slam" is increased (brightness too, unfortunately). Let me know. MrD.