I am a small speaker fan from way back and I have talked about them here too. I also prefer one small speaker as opposed to two in mono. It is not the same thing. It is better to sum the L+R signals outside your DAW too and somehow just organise it on its own output. Any spare headphone output will be perfect for feeding the speaker too.
I love hearing the mix coming through what I call a bottleneck like this down at low volume. It reveals so much about your mix. Once you get things balanced well on this your main speakers up loud will sound killer. The only thing it does not reveal so well is what the very low end is up to and reverbs. But loud playing on your mains will soon put you in the picture with those two things. With reverbs if you are hearing them well on your small speaker it sort of means the reverbs are a little loud! After a while you get to know what the correct reverb levels sound like on the small speaker.
It is ideal for setting vocal levels into the music, in fact almost the only way to get that right. Also if you have say two or three parts that are all sounding a bit too similar they don't fair well on the small speaker down low in mono. The parts all end up lined up behind each other and not being distinct enough. It forces you to get in there and tweak the parts so they stand out more so when listened to on the speaker in mono again. Then in stereo and up loud you will find the parts sound way better and more distinct from each other. Much easier to separate. And because you have done some work separating them they can be turned down too for maximum illusion minimum voltage.
I absolutely love it and spend far more time on it than the mains. And I monitor pretty low on it too like 75 dB SPL or less. It also means you are not pounding your ears or your neighbours too which is always a good thing. Even though I can monitor at extraordinary volumes I tend not too because of this.
It is a technique that goes back decades and has been used for years by many many respected engineers so it is well proven. It also provides another reference to hear your mixes on which is not a bad thing anyway. When your mixes sound great on the small mono speaker I find they translate perfectly to everything. A good mix is a good mix no matter what you hear it on. If your mixes are sounding different on every speaker you try you are doing something very wrong. The small speaker goes a long way to eliminate that scenario.
The very (deep) bottom end is still another situation and the small speaker does not help you too much in that area but I find once everything else is right though it is easily tamed and brought into line. I do that up loud on my mains and in my car because my car has a hyped low end. Commercial CD's still have this low punchy sound in the car that does not rattle my teeth. If my mix is rattling my teeth (in the car) I go back and sort it out usually. I have through trial and error found what the ideal low end curves are in the mastering stage. You must get you bottom end right ASAP though because it effects everything else so much. The mix is still the better place to get your bass and kick levels just right. It is surprising though still how much the small speaker will tell you about the bass and the kick, it is not useless in this regard at all. It tends to emphasise the upper harmonics of those things and if you can still hear the bass and kick nicely in the small speaker then you are well on the right path.