Lets first start by stating and hopefully agreeing on this: Your studio control room's design and acoustic layout is the most important factor, If it's off, it does not matter what speaker or monitor setup you have, everything will be off.
Even my professionally designed control room at Eclectica Studios had issues that I dealt with for years and ended up throwing a lot of money at it to correct it.
I talk with many other mixing engineers about how they monitor while mixing. Some use one set of speakers, other have 3 or 4 pair, some studios have studio monitors all staked up on top one other... Some swear by the age old practice of burning the mix to CD or cassette before that and listening to their work in their car. Dave Pensado talks about the small Aurotones he keeps to the side of him, right next to each other simply to hear the song.
We live in a full range world now.
My Studio monitors are Nuemann KH Series: the KH120 and Sub, fed by the Mytek Stereo192 DSD D/A. They sound amazing; clean, clear and correct.
Interestingly, for a minute I added a second set of 6.5" studio monitors, and had them sitting right next to the KH120s, But they killed the sound, the stereo field of the KH120s just closed up. Now when I see studio with speakers next to other speakers I wonder if they know what they are loosing by setting a big box next to a speaker they are listening too, or if they even noticed the stereo field disappearing and the sound closing up.
But really, what's the point of having lots of good sounding speakers that really sound basically the same when the consumer will be listening to your music on earbuds which have a completely different presentation or a Bose stereo system that is not setup to spec...
If I mix on speakers, as I do, and then switch to another pair of speakers, it confuses the situation as to my reference point of the tonalities of the mix..."Is there too much 600hz in the mix or is it just these speakers have more low mids than the first set?"
But popping in Earbuds or on a set of headphones will resets your brain and point of view for listening.
Aurotones are an interesting small 4.5" single speaker in a small box, and have for decades been "industry standard" for low-fi mix checking as they represented small boom boxes and TV speakers well. But it's almost 2016 (in a few weeks), people do not listen to music on boomboxes, or to their TVs using the built in speakers anymore. Boomboxes are now earbuds and headphones, TVs plug into Stereo and 5.1 Home systems. Car stereos are basically full range and decent nowadays, no more single speaker AM radios for automobile entertainment.
While interviewing engineers and artist for the "How We Listen" feature I wrote for Professional sound I spoke with Tony Harnell and him mentioning how he only listens to music on his earbuds nowadays, I realized, outside my studio walls, things really are changing...
So instead of a 2nd and 3rd pair of speakers or my car... I now check my mixes using a set of medium range earbuds and a set of decent headphones that sit between the Bose headphones and crappy Beats headphones that are so popular with fashion conscience. Since checking mixes and masters with earbuds and headphones earlier this year, my work is translating better than ever to all mediums.
So a few things to take away...
You have to work in an acoustically correct room. You need one set of REALLY good monitoring and know how they sound like you know your own soul. You need to know how speakers and the room sound together and there is no better way than to listen to music you know and love on them everyday.
Then, you need to be checking your mixes on what the consumers will be listening to your work on. For the most part, if its not a full range stereo, its earbuds and headphones.