I was hunting around for a reason a mic's output could be more towards one polarity than the other and came up with two possibilities.  One is if a circuit is clipping one side of the waveform (like a diode clipper) the other is a natural occurance as mentioned in this SOS article :
 
"The other element involved in this is that many acoustic sources inherently have a 'positive air pressure bias' because of the way the sound is generated. To talk or sing, we have to breathe out, and to play a trumpet, we have to blow air through the tubing. So, in these examples, there is inherently more energy available for the compression side of the sound wave than there is for the rarefaction side, and that can also contribute to an asymmetrical waveform."
 
Maybe your mic being close to your amp gets it into this situation, especially when loud ?.  It shouldn't be a problem if the signal half isn't short because it's clipped but rather the other side is tall because it's being blown on by the speaker cone (hopefully not too much so that it causes clipping on that side further down the line).  I can't tell from your pictures if your waveform is clipped on the Cube at guitar=10 or if it's just lopsided.  What do the peaks look like ?
 
Does this only happen when micing a guitar cab ?
 
I don't know what the effects on you gear would be if you approach 15a draw on your household circuit.  If the living room in question is normal, it's outlets are probably on the same circuit as outlets in adjacent rooms, like a dining room or entrance.  So there could be even more things plugged into your circuit.  You have a total of about 1500w to work with on a single 15a circuit so you could add up your equipment and see how you're doing.
 
 

Yeah, I think the equipment approached the limit. In the future, I think I'm going to get something like this...
http://www.amazon.com/CyberPower-CP1500AVRLCD-Intelligent-1500VA-Mini-Tower/dp/B000FBK3QK/ref=sr_1_2?ie=UTF8&qid=1427508815&sr=8-2&keywords=battery+ups+line+conditioner
 
Additionally, someone at MXL is helping me review the Mic - images of waveforms etc.
 
*** MXL is AWESOME ***
I'm totally surprised a human is reviewing this!!
 
It's a fair question about the Mic'ing of the cab. It's just an Amp. A 4x10. I put the Mix right in front of it to record it tonight. I did the same on the 2x12 when I recorded 2 weeks ago.
 
But the face of the Mic wasn't any closer to the speaker than the SM57 was or the Shure SM86. Both of those recorded the high volumes fine.
 
Is it possible this is a Mic positioning problem? It's too late in my neighborhood to give it a try now, but I will tomorrow. I'll try moving the mic back and away and see if that fixes things.

 
http://www.cyberpowersystems.com/products/ups-systems/intelligent-lcd-ups/CP1500AVRLCD.html
 
I don't think this can help with an overloaded circuit.  It has a capacity of 900w for three minutes.  I think it's only meant to keep a computer alive long enough to shut it down gracefully in the case of a power outage or during a very short interruption.
 
If you're drawing too much from the circuit, the only solution is to bring in another circuit.  You could use a 12ga extension from another room, like from a kitchen counter outlet which are normally on separate circuits.  But then you'll risk creating a ground loop by having two different grounds for your setup and having equipment on separate circuits jointed by audio cables (if that happens, you can lift the ground on one end of the audio cable, not the equipment supply ground).
 
 
 

@Paul P
Thanks so much for this tips. I tested the Mic a yard away from the Amp and it still clips the top of the wave form, starting at around 85dB using my phone dB meter. The amp is pretty loud, but not screaming. The room is pretty quiet. Other Mics like SM86 never has this (it's a condenser and it was right up to the grill of the amp).
 
I've pretty much decided I can't use the mic in the future which is too bad. It's only been used a few times and it just isn't worthy. As far as I can tell this problem is isolated to just this Mic.
 
There is some other intermittent issue that may be related to power supply (which is the clip I gave at the beginning of this post).
 
Anyone want to buy a MXL Guitar Cube?

 
I just came across this thread dealing with a MXL Drum Cube Pro :
Strange Waveform from MXL Drum Cube Pro
 
Look at this snare hit :
 

 
Looks familiar !  There's something pretty weird about these mics. They can't take high level in one direction.
 
If you like the sound of the mic, maybe you could still use it for quieter stuff.
 

I contacted Marshall through this link...
http://www.mxlmics.com/support/contact.php
 
And one of their people has let me know that the Mic should still be under warranty and that this is not expected behavior. It looks like they are going to honor the warranty.