I've just bought an Avantone Mixcube (the Aurotone replacement) and am wondering how you might use a single speaker in a studio setup.

I already have 3 pairs of 2 way speakers, 1 pair of 5" passive speakers, 1 pair of Adam A7X's and and a pair of JBL LSR 2328 as well as a JBL 2310 subwoofer.

I've never worked with a mono monitor so a little advice may go a long way.

Do I set it up directly in the middle (side to side) of my normal monitor position?
Does it even matter as long as I'm looking directly at it, as that would place it naturally in the middle of my listening field.

Any (relevant) tips are welcome.

I'd set it up in the middle, directly in front of your typical work/listening position and use a monitor controller so that you can switch to it with a single hardware button push.
 
I'm sure Jeff will jump in and give you proper guidelines as he has been (for a good reason) repeatedly recommending this single speaker setup (I personally use the avantones in a 2 speaker setup w/ mono switch as I have realized these little things can tell you so much about panning and stereo levels as well ...)

 
http://www.soundonsound.c...vantone-active-mixcube
 
Watch the video to get abetter idea mixing in Mono.
https://www.youtube.com/watch?v=57iD9HhVHOo
 
 

It can be anywhere really.  Mine is to the right as long as you are looking at it while checking a mix. 
 
The secret is to listen to it down at low volume too.  Do not pump it up. 
 
It is important to get a L+R mix into it.  I prefer to leave to leave the software alone and in stereo.  A spare headphone jack (on your interface or digital mixer etc) is the ideal place to pick up the stereo output from your DAW. My speaker is passive and I use a stereo hi fi amp with a mono switch and it works great.
 
For the active model a small passive 2 channel mixer will do the job usually as well.  The headphone jack will provide plenty of signal so any signal loss due to the summing won't be a problem.  Unless you have got a monitor controller of course.  You could even build a simple resistive mixer inside the connector that goes into the Avantone. 
 
What it does not show very well are reverbs and bass.  So it is important to switch over to your normal speakers and up louder for these checks.  The small speaker can make you increase the reverb levels a little only to find things are a little wet on your larger speakers at louder volumes.  So a good rule of thumb is to set reverb levels for just hearing it on your mains and you will usually be right.
 
But for everything else the small speaker is amazing.  Anything that is even slightly out on the big speakers will jump out at you on the small mono speaker.  There is a way to satisfy both very well.  It is also the best and only way to set vocal levels against the music too.  It is so fast doing this where as you can spend hours trying to do this on larger speakers.

I don't have the auratone (although I sat in front of them for decades) but I have a cheap small 3" PC USB powered speakers. They sound pretty good. So I have them side by side on top of my left monitor. They are fed from the headphone out of my NanoPatch (confirming what Jeff said). I hit the USB power button on my little USB multi switch and mute the mains. I put the master out in mono mode. 
I get a really good small speaker sound to check mixes in. 
It is so easy to flip between my mains that I check them a lot. 
The speakers are Altec Lansing like this but w/o the woofer:
https://www.amazon.com/Altec-Lansing-BXR1221-Speaker-System/dp/B0025VKUQ6
 

one of the things i really dig about using the IKM Arc2 software, 
is the ability to fold to mono at the touch of a button..
 
i routinely check my mixes along the way using this handy tool.
 
 

I have found from experience folding down a stereo mix into mono through your main speakers is nowhere near as good as feeding an L+R mix into a single mono speaker and a totally different speaker as well.
 
Mono means a single point source.  Something that two main speakers is not doing.  In fact even with two main speakers in mono you are still hearing a subtle form of stereo because the sound is arriving at both ears still slightly differently.  A single point source speaker eliminates this.
 
Hearing your mix on a totally different speaker too which has no low end or high end much is emphasising the mids much more so.  Something your main speakers also cannot do naturally.   By doing this and at low volume you are hearing all sorts of other aspects to your mix which your main speakers just cannot give you.  It is what I call the critical balance. 
 
I also find that once I get a mix sounding right on the mono speaker it simply translates perfectly everywhere too.  So you don't have to do all this checking on 5 other systems and in your car either either. 
 
 

it actually works pretty well, jeff.

batsbrew
it actually works pretty well, jeff.

 
I am sure it does too.  At least what you are doing in mono is hearing everything panned centre and that is still good for getting everything to work well together.  It will also still show you if some stuff is treading on other stuff which is good of course. It will still help to separate things as well a little better.
 
But I guarantee if you had the single Avantone setup you will love it and in time you would agree it is better than what you are doing now.  Unless you actually make the purchase and set it up properly you simply will never know.

I'm happy with the results I'm getting with the mono Mixcube too. With the Audient iD22, I can switch from the regular monitors to the Mixcube at the touch of a button. It really does reveal a lot about your mix.